On February 11, 2016, the LIGO (Laser Interferometric Gravitational-wave Observatory) Collaboration announced the detection of gravitational waves. For
the first time, scientists have observed ripples in the fabric of
spacetime called gravitational waves, arriving at the Earth from a
cataclysmic event in the distant universe. This confirms a major
prediction of Albert Einstein’s 1915 general theory of relativity
and opens an unprecedented new window onto the cosmos. 1)

Gravitational waves carry
information about their dramatic origins and about the nature of
gravity that cannot otherwise be obtained. Physicists have concluded
that the detected gravitational waves were produced during the final
fraction of a second of the merger of two black holes to produce a
single, more massive spinning black hole. This collision of two black
holes had been predicted but never observed.

The gravitational waves were
detected on September 14, 2015 at 09:51 UTC by both of the twin LIGO
detectors, located in Livingston, Louisiana, and Hanford, Washington,
USA. Each LIGO detector is 4 km long, the observatories are separated
by a distance of 3,002 km. As a gravitational wave passes through a
detector, it distorts spacetime such that one arm lengthens, and the
other shortens. By comparing the disturbances at the two detectors, the
scientists can confirm the direct detection of a gravitational wave. -
The LIGO Observatories are funded by the NSF (National Science
Foundation), and were conceived, built, and are operated by Caltech and
MIT. The discovery, accepted for publication in the journal Physical
Review Letters, was made by the LIGO Scientific Collaboration (which
includes the GEO Collaboration and the Australian Consortium for
Interferometric Gravitational Astronomy) and the Virgo Collaboration
using data from the two LIGO detectors. 2)

Based on the observed signals, LIGO
scientists estimate that the black holes for this event were about 29
and 36 times the mass of the sun, and the event took place 1.3 billion
years ago. About 3 times the mass of the sun was converted into
gravitational waves in a fraction of a second—with a peak power
output about 50 times that of the whole visible universe. By looking at
the time of arrival of the signals—the detector in Livingston
recorded the event 7 milliseconds before the detector in
Hanford—scientists can say that the source was located in the
Southern Hemisphere.

According to general relativity, a
pair of black holes orbiting around each other lose energy through the
emission of gravitational waves, causing them to gradually approach
each other over billions of years, and then much more quickly in the
final minutes. During the final fraction of a second, the two black
holes collide into each other at nearly one-half the speed of light and
form a single more massive black hole, converting a portion of the
combined black holes’ mass to energy, according to
Einstein’s formula E=mc2. This energy is emitted as a
final strong burst of gravitational waves. It is these gravitational
waves that LIGO has observed.

The new LIGO discovery is the first
observation of gravitational waves themselves, made by measuring the
tiny disturbances the waves make to space and time as they pass through
the Earth.

The discovery was made possible by
the enhanced capabilities of Advanced LIGO, a major upgrade that
increases the sensitivity of the instruments compared to the first
generation LIGO detectors, enabling a large increase in the volume of
the universe probed—and the discovery of gravitational waves
during its first observation run. The US National Science Foundation
leads in financial support for Advanced LIGO. Funding organizations in
Germany (Max Planck Society), the STFC (Science and Technology
Facilities Council) of the UK, and Australia (Australian Research
Council) also have made significant commitments to the project. Several
of the key technologies that made Advanced LIGO so much more sensitive
have been developed and tested by the German UK GEO collaboration.
Significant computer resources have been contributed by the AEI
Hannover Atlas Cluster, the LIGO Laboratory, Syracuse University, and
the University of Wisconsin- Milwaukee. Several universities designed,
built, and tested key components for Advanced LIGO: The Australian
National University, the University of Adelaide, the University of
Florida, Stanford University, Columbia University of the City of New
York, and Louisiana State University.

LIGO research is carried out by the
LIGO Scientific Collaboration (LSC), a group of more than 1000
scientists from universities around the United States and in 14 other
countries. More than 90 universities and research institutes in the LSC
develop detector technology and analyze data; approximately 250
students are strong contributing members of the collaboration. The LSC
detector network includes the LIGO interferometers and the GEO600
detector. The GEO team includes scientists at the Max Planck Institute
for Gravitational Physics (Albert Einstein Institute, AEI), Leibniz
Universität Hannover, along with partners at the University of
Glasgow, Cardiff University, the University of Birmingham, other
universities in the United Kingdom, and the University of the Balearic
Islands in Spain.

LIGO Interferometer

LIGO is a national US facility for
gravitational-wave research providing opportunities for the broader
scientific community to participate in detector development,
observation, and data analysis. The design and construction of LIGO was
carried out by a team of scientists, engineers, and staff at the
California Institute of Technology (Caltech) and the Massachusetts
Institute of Technology (MIT), and collaborators from over 80
scientific institutions world-wide that are members of the LSC (LIGO
Scientific Collaboration).

LIGO is the world's largest
gravitational wave observatory and a marvel of engineering. Comprising
two enormous laser interferometers located thousands of kilometers
apart, LIGO exploits the physical properties of light and of space
itself to detect and understand the origins of gravitational waves. 3)

LIGO's interferometers are the
largest ever built. With arms 4 km long, they are 360 times larger than
the one used in the Michelson-Morley experiment (which had arms 11 m
long).

Figure 1:
Basic Michelson interferometer with Fabry Perot cavities. Mirrors
placed near the beam splitter keep the laser contained within the arms.
This increases the distance traveled by the beams, greatly improving
LIGO's sensitivity to changes in arm length like those caused by
gravitational waves (image credit: Caltech, MIT)

This is particularly important in
the search for gravitational waves because the longer the arms of an
interferometer, the farther the laser travels, and the more sensitive
the instrument becomes. Attempting to measure a change in arm length
1,000 times smaller than a proton means that LIGO has to be more
sensitive than any scientific instrument ever built, so the longer the
better. But there are obvious limitations to how long one can build an
interferometer. Even with arms 4 km long, if LIGO's interferometers
were basic Michelsons they would still not be long enough to detect
gravitational waves...and yet they are. How is this possible?

This dilemma was fixed by adding something called "Fabry Perot cavities" to the basic Michelson design. The Figure 1
shows how a basic Michelson interferometer is modified to include Fabry
Perot cavities. It is created by adding mirrors near the beam splitter
that continually reflect parts of each laser beam back and forth within
the 4 km long arms about 280 times before they are merged together
again.

With Fabry Perot
cavities, LIGO's interferometer arms are effectively 1120 km long,
making them 144,000 times bigger than Michelson's original instrument!
This bit of 'mirror magic' greatly increases LIGO's sensitivity and
makes it capable of detecting changes in arm-length thousands of times
smaller than a proton, all while keeping the physical size of the
interferometer manageable.

Those familiar with telescopes will
recognize this effect. Increasing a telescope's focal length doesn't
just increase the magnification of any given eyepiece, it also
magnifies the tiniest vibrations making them visible in the eyepiece;
the longer the focal length, the smaller the vibration you see in the
eyepiece. In a telescope, these vibrations are unwelcome, but LIGO is
designed to feel them. And at effectively 1120 km long, LIGO's arms can
readily magnify the smallest conceivable vibrations enough that they
are measurable.

Power Boosted Laser: Length
isn't the only design factor important to LIGO's sensitivity; laser
power is too. While increasing length increases the interferometer's
sensitivity to vibrations, increasing laser power improves the
interferometer's resolution. The more photons that merge at the beam
splitter, the sharper the resulting interference pattern becomes,
making it 'easier' to recognize a gravitational wave signature.

But there's a problem here too. For
LIGO to operate at full sensitivity, its laser has to shine at 750 kW,
but LIGO's laser enters the interferometer at most at 200 W. And just
as it is impossible to build a 1120 km-long interferometer, building a
750 kW laser is also a practical impossibility. So how does LIGO boost
the power of its laser 3750 times without actually using more power?

Figure 2:
Basic Michelson Interferometer with Fabry Perot cavities and Power
Recycling mirror. LIGO's interferometers use multiple power recycling
mirrors, but for simplicity only one is shown in the diagram (image
credit: Caltech, MIT)

More mirrors!
Specifically, "power recycling" mirrors placed between the laser source
and the beam splitter. Like the beam splitter itself, the power
recycling mirror is only partly reflective (a 'one-way mirror'). Figure
2 shows schematically where such a mirror is located.

In a power recycling mirror, light
from the laser first passes through the mirror to reach the beam
splitter where it is split and directed down the arms of the
interferometer. The instrument is aligned so well that nearly all of
the reflected laser light from the arms follows a path back to the
recycling mirrors rather than to the photodetector. Laser light coming
from the ends of the arms is thereby reflected back into the
interferometer (hence 'recycling') where those photons add to the ones
just entering; more photons equals more power. This process greatly
boosts the power of the beam without needing to generate a 750 kW beam
at the outset.

The boost in power generated by this
recycling process enhances the interference pattern that results when
the two beams are superimposed after their long journey through the
interferometer. Since we expect to see particular interference patterns
when a gravitational wave passes by, the more prominent the pattern,
the easier it is for us to recognize and confirm that we have, in fact,
detected gravitational waves.

LIGO Technology (including updates)

• March 20, 2020: Scientists
have developed a new type of deformable mirror that could increase the
sensitivity of ground-based gravitational wave detectors. "In addition
to improving today's gravitational wave detectors, these new mirrors
will also be useful for increasing sensitivity in next generation
detectors and allow detection of new sources of gravitational waves,"
says Huy Tuong Cao. — Cao is a research team leader at the University of Adelaide node of the Australian Center of Excellence for Gravitational Waves Discovery (OzGrav ) and a member of the University's School of Physical Sciences. 4)

Figure 3: The illustration shows
the cross-section of a thermal bimorph mirror and its constituents.
Controlling the temperature of the mirror changes the curvature of the
reflected wavefront. Overlaid on the cross-section is the simulated
radial stress, showing a concentration of stress at the boundary of the
two layers, where the adhesive holds the structure together (image
credit: Huy Tuong Cao)

- Detectors such as the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO ) are set to benefit from the new technology.

- Advanced LIGO
measures faint ripples in space time called gravitational waves, which
are caused by distant events such as collisions between black holes or
neutron stars.

- Deformable mirrors, which are used
to shape and control laser light, have a surface made of tiny mirrors
that can each be moved, or actuated, to change the overall shape of the
mirror.

- Published in The Optical Society's
journal Applied Optics , Cao and colleagues have, for the first time,
made a deformable mirror based on the bimetallic effect in which a
temperature change is used to achieve mechanical displacement. 5)

- "Our new mirror provides a large
actuation range with great precision," says Cao. "The simplicity of the
design means it can turn commercially available optics into a
deformable mirror without any complicated or expensive equipment. This
makes it useful for any system where precise control of beam shape is
crucial."

- The new technology was conceived
by Cao and Aidan Brooks of LIGO as part of a visitor program between
the University of Adelaide and LIGO Laboratory, funded by the
Australian Research Council and National Science Foundation.

Building a better mirror

- Ground-based gravitational wave
detectors use laser light traveling back and forth down an
interferometer's two arms to monitor the distance between mirrors at
each arm's end. Gravitational waves cause a slight but detectable
variation in the distance between the mirrors.

- Detecting this tiny change
requires extremely precise laser beam steering and shaping, which is
accomplished with a deformable mirror.

- "We are reaching a point where the
precision needed to improve the sensitivity of gravitational wave
detectors is beyond what can be accomplished with the fabrication
techniques used to make deformable mirrors," says Cao.

- Most deformable mirrors use thin
mirrors to induce large amount of actuation, but these thin mirrors can
produce undesirable scattering because they are hard to polish. The
researchers designed a new type of deformable mirror using the
bimetallic effect by attaching a piece of metal to a glass mirror. When
the two are heated together the metal expands more than the glass,
causing the mirror to bend.

- The new design not only creates a
large amount of precise actuation but is also compact and requires
minimum modifications to existing systems. Both the fused silica
mirrors and aluminum plates used to create the deformable mirror are
commercially available. To attach the two layers, the researchers
carefully selected a bonding adhesive that would maximize actuation.

- "Importantly, the new design has
fewer optical surfaces for the laser beam to travel through,” Cao
says. "This reduces light loss caused by scattering or absorption of
coatings."

Precision characterization

- Creating a highly precise mirror
requires precision characterization techniques. The researchers
developed and built a highly sensitive Hartmann wave front sensor to
measure how the mirror's deformations changed the shape of laser light.

- "This sensor was crucial to our
experiment and is also used in gravitational detectors to measure
minute changes in the core optics of the interferometer," Cao says. "We
used it to characterize the performance of our mirrors and found that
the mirrors were highly stable and have a very linear response to
changes in temperature."

- The tests also showed that the adhesive is the main limiting factor for the mirrors' actuation range.

- The researchers are currently
working to overcome the limitation caused by the adhesive and will
perform more tests to verify compatibility before incorporating the
mirrors into Advanced LIGO.

• Designing instruments like LIGO's interferometers, capable of measuring a distance on the order of 10-19
m required inventing and refining innovative technology. Most of
LIGO’s most impressive technology resides in its seismic
isolation systems (which remove unwanted vibrations), vacuum systems
(to make sure the laser light is kept pure), optics components (to
preserve laser light and laser power), and computing infrastructure (to
handle the mindboggling amount of data that LIGO collects). These
systems are like LIGO's internal organs. If any one fails, the whole
instrument suffers (Ref. 3).

While each of these components is a
feat of engineering in itself, without working seamlessly together,
LIGO, as a single multifaceted instrument could never achieve its
scientific goals. A basic overview of each of LIGO's critical systems
is provided.

Seismic Isolation:
LIGO’s greatest strength is also its greatest weakness. Since
LIGO is designed to sense the smallest conceivable motions of mirrors
caused by the passage of a fleeting gravitational wave, it is also
extremely sensitive to all vibrations near (such as trucks driving on
nearby roads) and far (earthquakes on the other side of the world).
Without taking extraordinary measures, any number of Earthly vibrations
could move LIGO’s primary mirrors (LIGO scientists call them
“test masses”) enough to hide a gravitational wave signal.
Isolating LIGO from as much environmental vibration as possible is the
linchpin in LIGO’s quest to feel gravitational waves. To that
end, LIGO uses multiple means of eliminating vibration falling into two
broad categories: “active” and “passive”
damping systems.

Active Damping:
The first line of defense against unwanted vibration is LIGO’s
“active” damping system. The Internal Seismic Isolation
(ISI) system consists of devices that sense ground movements and then
deliberately perform counter movements to eliminate them, keeping the
instrument motion-free.

Obviously, the local environment is
always creating vibrations of one form or another. LIGO’s ISI
system contains sensors designed to feel different frequencies caused
by different environmental vibrations. These sensors work side-by side
and send their signals to a computer that combines the effects of all
of these motions and then generates a net counter-motion to cancel all
of the vibrations simultaneously. It is very similar to how
noise-canceling headphones work!

Passive Damping: LIGO’s
passive damping system holds the all-important mirrors perfectly still
through a 4-stage pendulum called a "quad". In the quad, LIGO’s
test masses (its mirrors) are suspended at the end of four pendulums by
0.4 mm thick fused-silica (glass) fibers. The "Main Chain" side faces
the laser beam, while the "Reaction mass" side helps to keep the test
mass steady from noise not associated with sources from space. This
configuration absorbs any movement not completely canceled out by the
active (ISI) system. The sheer mass of the suspension components (each
mirror weighs 40 kg) also helps to prevent motion of the mirrors thanks
to the Law of Inertia.

Working together, these active and
passive vibration damping systems ensure that LIGO's lasers and mirrors
are isolated from as much external noise and vibration as is physically
possible.

Vacuum: LIGO contains one of
the largest and purest sustained vacuums on Earth. In volume, it is
surpassed only by the Large Hadron Collider in Switzerland. The
atmospheric pressure inside LIGO's vacuum tubes is one-trillionth that
of air pressure at sea level. LIGO needs such a good vacuum for two
reasons:

1) Air – or even just a few
molecules of air – can create noise which masks the tiny changes
in distance between mirrors we seek to detect. One way this can happen
is due to Brownian motion, or the fact that everything with a
temperature above absolute zero is moving with heat energy. Molecules
of air hitting the mirrors can cause them to move, masking the
gravitational waves. Another way residual air in the laser light path
causes problems is similar to the shimmering one sees over a hot road
– air has an ‘index of refraction’, and like a glass
lens, can change the path of light. Even just a few molecules passing
through the laser light beam can affect the apparent distance between
the mirrors, once again masking the subtle effects of a passing
gravitational wave.

2) The second critical reason for
operating in a vacuum is to eliminate the chances that dust will drift
into the path of the laser, or worse, onto a mirror causing some of the
light to scatter (i.e., be reflected in some random direction away from
its path).

Without operating in such a high
quality vacuum, LIGO’s lasers would be absorbed and deflected
enough to create unwanted interference patterns that might be mistaken
for or drown out a signal from a gravitational wave. Losing any laser
photons could cripple LIGO’s ability to detect gravitational
waves.

Creating such a large volume of
empty space on Earth was no easy task. Many techniques were used to
remove all the air and other molecules from LIGO’s vacuum tubes:

• The tubes were heated to between 150ºC and 170ºC for 30 days to drive out residual gas molecules.

• Turbo-pump vacuums (like
little jet engines that create suction instead of thrust) sucked out
the bulk of the air contained in the tubes

• Ion pumps then extract
individual remaining gas molecules by electrically charging them and
then attracting them away with opposite charge, like a magnet. Since
the metal inside the vacuum chamber is always emitting some gaseous
molecules ("outgassing"), these pumps operate continuously in order to
maintain the pristine vacuum inside the tubes.

It took 40 days (1100 hours) to remove all 10,000 m3
of air and other residual gases from each of LIGO’s vacuum tubes
to reach an air pressure one-trillionth that at sea level.

Optics System: LIGO's optics
system consists of lasers, a series of mirrors, and a photodetector (a
device that measures varying light levels). In order to measure a
movement thousands of times smaller than a proton, LIGO's optical
components must operate harmoniously and with unprecedented precision.
It all begins with the main laser.

LASER: We
all encounter lasers daily in laser pointers, in cat toys, or in the
barcode scanners at the grocery store. Because of their omnipresence
most of us tend to take them for granted without really knowing how
they work. If you're one of those people, you are not alone! If you
want to know how they work, Cambridge University’s “Naked
Science Scrapbook” video, “How do lasers work?”
provides a fun, easy-to-understand explanation. Once you grasp the
basic principles, understanding LIGO's laser is a snap!

The first thing to understand is
that the word, "laser" is actually an acronym for “Light
Amplification by the Stimulated Emission of Radiation”. This
means that "laser" refers to a process more than a thing, but most of
us now use the term ubiquitously to refer to the device that generates
the laser beam, or the beam itself. This might be splitting hairs, but
when it comes to understanding LIGO's laser, it's an important
distinction.

The heart of LIGO is its 200 W laser
beam. But you might be surprised to know that the beam doesn't start
out at 200 W. It actually takes four steps to amplify its power and
refine its wavelength to a level of precision never before seen in a
laser of this kind.

The very first glimmer of light that
ultimately becomes LIGO's powerful laser emerges from a laser diode,
which uses electricity to generate an 808 nanometer (nm) near-infrared
beam of about 4 W. This is the same kind of device that a typical laser
pointer uses. While 4 W doesn't seem like a lot, the laser in the
average laser pointer shines at less than 5 mW (milliWatt). So LIGO's 4
W beam is 800 times more powerful than that laser pointer you use to
entertain your pet!

The second step in boosting LIGO's
laser up to 200 W occurs when the 4 W beam enters a device called a
Non-Planar Ring Oscillator (NPRO). The NPRO consists of a remarkably
small boat-shaped crystal about the size of a pinky fingernail! The 4 W
beam bounces around inside this crystal and stimulates the emission
(the S and E in LASER) of a 2 W beam with a wavelength of 1064 nm (in
the invisible infrared part of the spectrum).

Step three in LIGO's laser
amplification occurs when the now 2 W beam enters another amplifying
device that boosts the 1064 nm beam from 2 to 35 W. Getting from 35 W
to 200 W requires a different kind of device, however. So the 35 W beam
is sent through a device called a High Powered Oscillator (HPO), which
performs further amplification and refinement, and generates the 200 W
beam of pristine "lased" light. This is the beam that ultimately enters
LIGO's interferometer.

This multi-stage amplified laser is
required for LIGO because of its need to continually produce a pristine
single wavelength of light. In fact, LIGO's laser is the most stable
ever made to produce light at this wavelength. This stability is one of
several factors critical for LIGO's ability to detect gravitational
waves.

Mirrors: LIGO’s mirrors
are the highest quality available, both in material and shape. Made of
very pure fused silica glass, they absorb just one in 3-million photons
that hit them. This is important because it means that most of the
laser light is reflected. Reflecting most of the light means that the
mirrors are not prone to heating. Too much heat from the laser could
alter the mirror shapes enough that they degrade the quality of the
laser light. Any degradation would hamper LIGO's ability to distinguish
a gravitational wave from environmental noise. Lastly, the highly
reflective surface preserves laser power — again, the more power,
the better LIGO's resolution.

The mirrors also refocus the laser,
keeping the beam traveling coherently, meaning that it doesn't spread
out as it travels throughout its multiple reflections before
encountering the photodetector.

Finally, the mirrors were polished
so precisely that the difference between the theoretical design (the
perfect mirror shape as designed on a computer) and the actual polished
mirror surface is measured in atoms! This is critical because, with all
the reflections it goes through, each laser in each arm travels about
1120 km before being merged with its partner and reflected one last
time to the photodetector. Maintaining the stability and purity of the
laser light is one of LIGO's biggest challenges.

Computation and Data Collection: Computers are required both to run the LIGO instruments and to process the data that it collects.

When it is in 'observing' mode, LIGO
generates terabytes (1000's of gigabytes) of data every day. All of
this information must be transferred to a network of supercomputers for
storage and archiving. Such supercomputers are located at each of the
observatories, at Caltech, at MIT, and at various other institutions.
Once the data is secured, scientists can use customized computer
programs to scour the data for gravitational waves.

The amount of data LIGO collects is
as incomprehensively large as gravitational wave signals are small.
LIGO's archive already holds the equivalent over over 1-million DVDs of
data and will add the equivalent of about 178-thousand DVDs each year
to its archive. In actual numerical terms, the data archive at Caltech
holds over 4.5 Petabytes (PB, or 4.5 x 1015bytes) of data,
and will grow at a rate of about 0.8 PB (800 terabytes) per year.
What's a petabyte? If you wanted to count up to a petabyte by counting
one byte per second, it would take you 35.7 million years to reach one
petabyte!

Storing information is one thing;
processing it is another. Processing and analyzing all of LIGO's data
requires a vast computing infrastructure. For LIGO's first observing
run in 2015, the LIGO Lab will provide 35 MSU (million service units)
worth of computing cycles/time. This is equivalent to running a modern
4-core laptop computer for 1,000 years! The amount of computing time is
expected to grow by a factor of 10 to around 400 MSU by the time LIGO
has completed its third observing run.

LIGO Facility

LIGO's original instrument, a largely 'proof of concept' model dubbed "Initial LIGO",
engaged in "science observations" from 2002 to 2010. No detections were
made in that time, but enormous strides in detector engineering were
achieved as a result of what was learned during that initial run. 2010
marked the end of the Initial LIGO project, and as planned, between
2010 and 2014, both interferometers were completely overhauled to
incorporate much more sophisticated engineering.

This "Advanced LIGO" project
successfully improved the capabilities of the detectors, and within
days of turning on the new and improved instruments, LIGO made its
first detection of gravitational waves, generated by a pair of
colliding black holes some 1.3 billion light years away. Since that
historic day, LIGO's engineers have continued to improve the detectors'
sensitivities. The success of these improvements is evidenced by the
many more gravitational wave detections that have since been made.
Ultimately, with continued refinement and upgrading, Advanced LIGO's
detectors will achieve a sensitivity 10 times greater than Initial
LIGO, bringing 1000 times more galaxies into LIGO's observational
range.

Light is—or has been up until
now—the only way to study objects in the universe (actually the
entire electromagnetic spectrum). This includes everything from the
Moon, all the way out to the most distant objects ever observed.
Astronomers and astrophysicists use observatories that can see in not
only visible light, but in all other parts of the electromagnetic
spectrum, to study objects in the universe. And we’ve learned an
awful lot. But things will change with this announcement. 6)7)

Gravitational waves are a new way to study notoriously difficult things to observe like black holes and neutron stars. Black
holes emit no light at all, and their characteristics and properties
are inferred from cause and effect relationships with objects near
them. But the detection of gravitational waves holds the promise of
answering questions about black holes, neutron stars, and even the
early days of our universe, including the Big Bang.

It’s almost
impossible to overstate the magnitude of this discovery. Once we
understand how to better detect and observe gravitational waves, we may
come to a whole new understanding of the universe, and we may look back
on this day as truly ground-breaking and revolutionary.

Some background:

Experiments to detect gravitational
waves began with Weber and his resonant mass detectors in the 1960s,
followed by an international network of cryogenic resonant detectors.
Interferometric detectors were first suggested in the early 1960s and
the 1970s (Ref. 2). A study of the noise
and performance of such detectors, and further concepts to improve
them, led to proposals for long-baseline broadband laser
interferometers with the potential for significantly increased
sensitivity.

The existence of gravitational waves
was first demonstrated in the 1970s and 80s by Joseph Taylor, Jr., and
colleagues. Taylor and Russell Hulse discovered in 1974 a binary system
composed of a pulsar in orbit around a neutron star. Taylor and Joel M.
Weisberg in 1982 found that the orbit of the pulsar was slowly
shrinking over time because of the release of energy in the form of
gravitational waves. For discovering the pulsar and showing that it
would make possible this particular gravitational wave measurement,
Hulse and Taylor were awarded the Nobel Prize in Physics in 1993.

• October 3, 2017: The LIGO
Laboratory, comprising LIGO Hanford, LIGO Livingston, Caltech, and MIT
are excited to announce that LIGO’s three longest-standing and
greatest champions have been awarded the 2017 Nobel Prize in Physics:
Barry Barish and Kip Thorne of Caltech and Rainer Weiss of MIT. The
LIGO Scientific Collaboration is absolutely delighted to congratulate
Rainer Weiss, Barry Barish, and Kip Thorne on winning the 2017 Nobel
Prize in Physics. Weiss and Thorne are two of the founders of the LIGO
project. Barish was the Principal Investigator of LIGO from 1994 to
2005, during the period of its construction and initial operation. 8)

Figure 5: Image credit: LIGO/Caltech/MIT

Decades in the Making:
While from the outside, it may seem surprising that this Nobel Prize
was awarded a scant 2 years after the discovery of gravitational waves
(often, Nobel Prizes are awarded many years after discoveries), for the
three laureates, it actually comes at the culmination of decades of
effort. LIGO may have only recently detected gravitational waves, but
its journey to doing so began nearly 45 years ago.

The very idea for LIGO came to
Rainer Weiss in the early 1970’s when, as associate professor of
physics at MIT, he had to find a way to explain gravitational waves (a
prediction of general relativity) to his students. In an interview with
MIT news writer, Jennifer Chu, Weiss recalled his revelation:

“That was my quandary at the
time, and that’s when the invention was made. I said,
‘What’s the simplest thing I can think of to show these
students that you could detect the influence of a gravitational wave?
... The obvious thing to me was, let’s take freely floating
masses in space and measure the time it takes light to travel between
them. The presence of a gravitational wave would change that time.
[Later] knowing what you could do with lasers, I worked it out: Could
you actually detect gravitational waves this way? And I came to the
conclusion that yes, you could detect gravitational waves ....”

Sometime later, in 1972, Weiss
carefully thought through and wrote down his idea, subsequently
publishing it as a paper titled, "Electromagnetically Coupled Broadband
Gravitational Antenna" . In this paper, Weiss described in great
detail, the design and promise of using laser interferometry to detect
gravitational waves. Within its 22 pages, the paper laid out the
blueprint for the Laser Interferometer Gravitational-Wave Observatory
(at the time, Weiss called it an antenna.) — And with that, LIGO
was born (at least on paper).

Today, no one disputes the fact that LIGO owes its very existence to Rainer Weiss.

Transforming LIGO from concept to
reality, however, would take another 20 years. There is no way that one
man, even the enigmatic Rai Weiss, could get LIGO built by himself. So
early on in those first two decades of navigating a circuitous path,
trying to keep the idea of a radical scientific experiment alive, Rai
found an ideal partner. In 1975, a fateful meeting between Weiss and
Kip Thorne of Caltech would set into motion the development of one of
the most complicated and risky scientific experiments ever conceived.
Thorne, already a highly respected, accomplished, and influential
theoretical physicist (with expertise in gravitational waves since the
late 1960s), commanded a level of respect among peers, colleagues,
research groups, and funding agencies that was unequalled. Kip’s
contributions in setting the astrophysics goals were central to the
design of the Observatories and the first instruments, and his presence
lent a high level of validity and credence to the idea.

As much as Rai is responsible for
conceiving of LIGO, Kip Thorne is equally responsible for convincing
countless others of LIGO’s potential for success. As a result of
his interactions with Weiss, Thorne convinced Caltech to create a
gravitational-wave research group. The group would be led by Ron
Drever, from Glasgow University (Drever, considered a co-founder of
LIGO, would go on to co-invent the Pound–Drever–Hall technique
for laser stabilization, which was critical to LIGO’s ability to
detect gravitational waves). Sadly, Drever passed away earlier this
year, but not before he learned of LIGO’s success.

Thus was formed the LIGO
triumvirate. With Rai’s vision and original idea, Thorne’s
brilliant theoretical physics mind, Drever’s brilliant
engineering, and all of their remarkable capacities to champion the
effort, nothing would stand in the way of LIGO’s evolution from
concept to reality.

In 1989, Weiss and Thorne, along
with Ron Drever, Fred Raab, and Robbie Vogt, submitted a proposal for
LIGO to the U.S. National Science Foundation (NSF) which has
enthusiastically supported this effort from the start. The proposal
included Rai’s original design for the instrument, updated to
include engineering and technology innovations that had occurred in the
intervening 17 years. To their credit, and at great risk, the NSF
approved the proposal. In 1994 construction began on the twin LIGO
detectors in Hanford, Washington and Livingston, Louisiana.

That’s when Barry Barish
joined the trio. Barish became LIGO Principal Investigator in 1994,
helping the growing cadre move from a small intensely focused group
cooking up basic ideas to a large and broad team that could actually
deliver the Observatories and hardware. Barry came with knowledge of
how to build a project that would succeed, but more importantly, he
came with vision and an incredible ability to strategize and make the
science happen. His insights led not only to construction of the
Observatories and the initial detectors, but also to the creation of
the LIGO Scientific Collaboration and to the successful proposal to
build Advanced LIGO—the instrument that would finally make the
first detection. Barry was the perfect complement to Kip and Rai to
bring LIGO to success.

The rest is history: Another
21 years would pass before the efforts of Rai Weiss, Kip Thorne, and
Barry Barish finally paid off. On September 14, 2015, LIGO detected the
gossamer flutters of spacetime created by the merging of two massive
black holes some 1.3 billion light years away. Nearly half-a-century
after its conception, LIGO had fulfilled its destiny.

So, while this Nobel Prize is being
awarded barely two years after LIGO’s historic detection, it
acknowledges 45 years of effort: from conception, through design,
planning, testing and prototyping, through decades of research and
engineering, invention and innovation, advances in computing, lasers,
and optics, and especially the championing and advocating of three
remarkable men: Barry Barish, Kip Thorne, and Rai Weiss. There can be
no doubt; their recognition by the Nobel Committee is well earned.
— Congratulations to you all!

• By the
early 2000s, a set of initial detectors was completed, including TAMA
300 in Japan, GEO600 in Hannover, Germany, the LIGO (Laser
Interferometer Gravitational-Wave Observatory) in the United States,
and Virgo in Italy. Combinations of these detectors made joint
observations from 2002 through 2011, setting upper limits on a variety
of gravitational-wave sources while evolving into a global network. In
2015, Advanced LIGO became the first of a significantly more sensitive
network of advanced detectors to begin observations.

Status of the LIGO-Virgo Network gravity-wave detection

• May 22, 2020:
Gravitational-wave researchers at the University of Birmingham have
developed a new model that promises to yield fresh insights into the
structure and composition of neutron stars. 9)

Figure 6: The results from a
numerical relativity simulation of two merging neutron stars similar to
GW170817 (image credit: University of Birmingham)

- The model shows that vibrations,
or oscillations, inside the stars can be directly measured from the
gravitational-wave signal alone. This is because neutron stars will
become deformed under the influence of tidal forces, causing them to
oscillate at characteristic frequencies, and these encode unique
information about the star in the gravitational-wave signal.

- This makes
asteroseismology - the study of stellar oscillations - with
gravitational waves from colliding neutron stars a promising new tool
to probe the elusive nature of extremely dense nuclear matter.

- Neutron stars are the ultradense
remnants of collapsed massive stars. They have been observed in the
thousands in the electromagnetic spectrum and yet little is known about
their nature. Unique information can be gleaned through measuring the
gravitational waves emitted when two neutron stars meet and form a
binary system. First predicted by Albert Einstein, these ripples in
spacetime were first detected by the Advanced Laser Interferometer
Gravitational Wave Observatory (LIGO) in 2015.

- By utilizing the gravitational
wave signal to measure the oscillations of the neutron stars,
researchers will be able to discover new insights into the interior of
these stars. The study is published in Nature Communications. 10)

- Dr Geraint Pratten, of the
University of Birmingham's Gravitational Wave Institute, is lead author
of the study. He explained: "As the two stars spiral around each other,
their shapes become distorted by the gravitational force exerted by
their companion. This becomes more and more pronounced and leaves a
unique imprint in the gravitational wave signal.

- "The tidal forces acting on the
neutron stars excite oscillations inside the star giving us insight
into their internal structure. By measuring these oscillations from the
gravitational-wave signal, we can extract information about the
fundamental nature and composition of these mysterious objects that
would otherwise be inaccessible."

- The model developed by the team
enables the frequency of these oscillations to be determined directly
from gravitational-wave measurements for the first time. The
researchers used their model on the first observed gravitational-wave
signal from a binary neutron star merger - GW170817.

- Co-lead author, Dr Patricia
Schmidt, added: "Almost three years after the first gravitational-waves
from a binary neutron star were observed, we are still finding new ways
to extract more information about them from the signals. The more
information we can gather by developing ever more sophisticated
theoretical models, the closer we will get to revealing the true nature
of neutron stars."

- Next generation gravitational wave
observatories planned for the 2030s, will be capable of detecting far
more binary neutron stars and observing them in much greater detail
than is currently possible. The model produced by the Birmingham team
will make a significant contribution to this science.

- "The information from this initial
event was limited as there was quite a lot of background noise that
made the signal difficult to isolate," says Dr Pratten. "With more
sophisticated instruments we can measure the frequencies of these
oscillations much more precisely and this should start to yield some
really interesting insights."

• April 20, 2020: The
expectations of the gravitational-wave research community have been
fulfilled: gravitational-wave discoveries are now part of their daily
work as they have identified in the past observing run, O3, new
gravitational-wave candidates about once a week. But now, the
researchers have published a remarkable signal unlike any of those seen
before: GW190412 is the first observation of a binary black hole merger
where the two black holes have distinctly different masses of about 8
and 30 times that of our Sun. This not only has allowed more precise
measurements of the system’s astrophysical properties, but it has
also enabled the LIGO/Virgo scientists to verify a so far untested
prediction of Einstein’s theory of general relativity. 11)

- “For the very first time we
have ‘heard’ in GW190412 the unmistakable
gravitational-wave hum of a higher harmonic, similar to overtones of
musical instruments,” explains Frank Ohme, leader of the
Independent Max Planck Research Group “Binary Merger Observations
and Numerical Relativity” at the Max Planck Institute for
Gravitational Physics (Albert Einstein Institute; AEI) in Hannover.
“In systems with unequal masses like GW190412 – our first
observation of this type – these overtones in the
gravitational-wave signal are much louder than in our usual
observations. This is why we couldn’t hear them before, but in
GW190412, we finally can.” This observation once again confirms
Einstein’s theory of general relativity, which predicts the
existence of these higher harmonics, i.e. gravitational waves at two or
three times the fundamental frequency observed so far.

- “The black holes at the
heart of GW190412 have 8 and 30 times the mass of our Sun,
respectively. This is the first binary black-hole system we have
observed for which the difference between the masses of the two black
holes is so large!” says Roberto Cotesta, a PhD student in the
“Astrophysical and Cosmological Relativity” division at the
AEI in Potsdam. “This big mass difference means that we can more
precisely measure several properties of the system: its distance to us,
the angle we look at it, and how fast the heavy black hole spins around
its axis.”

Figure 8: Numerical simulation of
two black holes that inspiral and merge, emitting gravitational waves.
One black hole is 3.5x more massive than the other and spins, which
makes the orbit precess. The simulated gravitational wave signal is
consistent with the observation made by the LIGO and Virgo
gravitational wave detectors on April 12th, 2019 (GW190412), [video
credit: N. Fischer, H. Pfeiffer, A. Buonanno (Max Planck Institute for
Gravitational Physics), Simulating eXtreme Spacetimes project]

Details on the visualization:
* The „apparent horizon“ of the black holes in the
simulation are shown in black. At 1:09 the simulation finds an
enveloping apparent horizon that signals the two black holes have
merged.
* The gravitational radiation is translated to colors around the black
holes. The colors transition from blue, representing weak radiation, to
red, representing strong radiation. Specifically, the coloring
represents the real part of the gravitational wave strain with its
inverse radial scaling removed for visualization. The strain is
computed from the simulation’s extrapolated waveform, which is
shown at the bottom of the screen.

A Signal Like None Before

- GW190412 was observed by both LIGO
detectors and the Virgo detector on 12th of April 2019, early during
the detectors’ third observation run O3. Analyses reveal that the
merger happened at a distance of 1.9 to 2.9 billion light-years from
Earth. The new unequal mass system is a unique discovery, since all
binaries observed previously by the LIGO and Virgo detectors consisted
of two roughly similar masses.

- Unequal masses imprint themselves
on the observed gravitational-wave signal, which in turn allow
scientists to more precisely measure certain astrophysical properties
of the system. The presence of higher harmonics makes it possible to
break an ambiguity between the distance to the system and the angle we
look at its orbital plane; therefore these properties can be measured
with higher precision than in equal-mass systems without higher
harmonics.

- “During O1 and O2, we have
observed the tip of the iceberg of the binary population composed of
stellar-mass black holes,” says Alessandra Buonanno, director of
the “Astrophysical and Cosmological Relativity” division at
the AEI in Potsdam and College Park professor at the University of
Maryland. “Thanks to the improved sensitivity, GW190412 has begun
to reveal us a more diverse, submerged population, characterized by
mass asymmetry as large as 4 and black holes spinning at about 40% the
possible maximum value allowed by general relativity,” she adds.

- AEI researchers contributed to
detecting and analyzing GW190412. They have provided accurate models of
the gravitational waves from coalescing black holes that included, for
the first time, both the precession of the black-holes’ spins and
multipole moments beyond the dominant quadrupole. Those features
imprinted in the waveform were crucial to extract unique information
about the source’s properties and carry out tests of general
relativity. The high-performance computer clusters
“Minerva” and “Hypatia” at AEI Potsdam and
“Holodeck” at AEI Hannover contributed significantly to the
analysis of the signal.

Testing Einstein’s theory

- LIGO/Virgo scientists also used
GW190412 to look for deviations of the signals from what
Einstein’s general theory of relativity predicts. Even though the
signal has properties unlike all others found so far, the researchers
could find no significant departure from the general-relativistic
predictions.

An Improved International Network of Detectors Using Squeezed Light

- This discovery is the second
reported from the third observation run (O3) of the international
gravitational-wave detector network. Scientists at the three large
detectors have made several technological upgrades to the instruments.

- “During O3, squeezed light
was used to enhance the sensitivity of LIGO and Virgo. This technique
of carefully tuning the quantum-mechanical properties of the laser
light was pioneered at the German-British detector GEO600,”
explains Karsten Danzmann, director at the AEI Hannover and director of
the Institute for Gravitational Physics at Leibniz University Hannover.
“The AEI is leading the world-wide efforts to maximize the degree
of squeezing, which has already improved the sensitivity of the GEO600
detector by a factor of two. Our advances in this technology will
benefit all future gravitational-wave detectors.”

2 done, 54 on the to-do list

- The detector network has issued
alerts for 56 possible gravitational-wave events (candidates) in O3
(April 1, 2019 to March 27, 2020 with an interruption for upgrades and
commissioning in October 2019). Out of these 56, one other confirmed
signal, GW190425, has already been published. LIGO and Virgo scientists
are examining all remaining 54 candidates and will publish all those
for which detailed follow-up analyses confirm their astrophysical
origin.

- The observation of GW190412 means
that similar systems are probably not as rare as predicted by some
models. Therefore, with additional gravitational-wave observations and
growing event catalogues in the future, more such signals are to be
expected. Each of them could help astronomers better understand how
black holes and their binary systems are formed, and shed new light on
the fundamental physics of space-time. 12)

• January 6, 2020: A pair of
neutron stars has been found with an unusually high mass. On April 25,
2019, the LIGO Livingston Observatory picked up what appeared to be
gravitational ripples from a collision of two neutron stars. LIGO
Livingston is part of a gravitational-wave network that includes LIGO
(the Laser Interferometer Gravitational-wave Observatory), funded by
the National Science Foundation (NSF) and the European Virgo detector.
Now, a new study confirms that this event was indeed likely the result
of a merger of two neutron stars. This would be only the second time
this type of event ever been observed in gravitational waves. 13)

- The first such observation,
which took place in August of 2017, made history for being the first
time that both gravitational waves and light were detected from the
same cosmic event. The April 25 merger, by contrast, did not result in
any light being detected. However, through an analysis of the
gravitational-wave data alone, researchers have learned that the
collision produced an object with an unusually high mass.

- "From conventional observations
with light, we already knew of 17 binary neutron star systems in our
own galaxy and we have estimated the masses of these stars," says Ben
Farr, a LIGO team member based at the University of Oregon. "What's
surprising is that the combined mass of this binary is much higher than
what was expected."

- "We have detected a second event
consistent with a binary neutron star system and this is an important
confirmation of the August 2017 event that marked an exciting new
beginning for multi-messenger astronomy two years ago," says Virgo
spokesperson Jo van den Brand, a professor at Maastricht University,
Nikhef, and VU University Amsterdam in the Netherlands. Multi-messenger
astronomy occurs when different types of signals, such as those based
on gravitational waves and light, are witnessed simultaneously.

- The study, ('GW190425: Observation of a Compact Binary Coalescence with Total Mass~3.4Mo')
submitted to The Astrophysical Journal Letters, is authored by an
international team comprised of the LIGO Scientific Collaboration and
the Virgo Collaboration, the latter of which is associated with the
Virgo gravitational-wave detector in Italy. The results were presented
at a press briefing today, January 6, at the 235th meeting of the
American Astronomical Society, held in Honolulu, Hawaii.

- Neutron stars are the remnants of
dying stars that undergo catastrophic explosions as they collapse at
the end of their lives. When two neutron stars spiral together, they
undergo a violent merger that sends gravitational shudders through the
fabric of space and time.

- The August 17 neutron star merger
was witnessed by both LIGO detectors, one in Livingston, Louisiana, and
one in Hanford, Washington, together with a host of light-based
telescopes around the world (neutron star collisions produce light,
while black hole collisions are generally thought not to do so). This
merger was not clearly visible in the Virgo data, but that fact
provided key information that ultimately pinpointed the event's
location in the sky.

- The April 2019 event was first
identified in data from the LIGO Livingston detector alone. The LIGO
Hanford detector was temporarily offline. The event took place more
than 500 million light-years from Earth, and so was too faint to be
detected with Virgo's current sensitivity. However, using the
Livingston data, combined with information derived from Virgo's lack of
a detection, the team narrowed the location of the event to a patch of
sky more than 8,200 square degrees in size, representing about 20
percent of the sky. For comparison, the August 2017 event was narrowed
to a region of just 16 square degrees, or 0.04 percent of the sky.

- "This is our first published event
for a single-observatory detection," says Caltech's Anamaria Effler (BS
'06), a scientist who works at LIGO Livingston. "But Virgo made a
valuable contribution. We used information about its non-detection to
tell us roughly where the signal must have originated from."

- The LIGO data reveal that the
combined mass of the merged bodies is about 3.4 times that of the mass
of our sun. Previously detected neutron star collisions produced final
masses of no more than 2.9 times that of the sun. One possibility for
the unusually high mass is that the collision took place not between
two neutron stars but between a neutron star and a black hole, since
black holes are heavier than neutron stars. But if this were the case,
the black hole would have to be exceptionally small for its class.
Instead, the scientists believe it is much more likely that LIGO
witnessed a shattering of two neutron stars.

- "What we know from the data are
the masses, and the individual masses most likely correspond to neutron
stars. However, as a binary neutron star system, the total mass is much
higher than any of the other known galactic neutron star binaries,"
says Surabhi Sachdev (MS '17, PhD '19), a LIGO team member based at
Penn State. "And this could have interesting implications for how the
pair originally formed."

- Neutron star pairs are thought to
form in two possible ways. They might form from binary systems of
massive stars that each end their lives as neutron stars, or they might
arise when two separately formed neutron stars come together within a
dense stellar environment. The LIGO data for the April 25 event do not
indicate which of these scenarios is more likely, but they do suggest
that more data and new models are needed to explain the merger's
unexpectedly high mass.

• September 9, 2019: The final
chapter of the historic detection of the powerful merger of two neutron
stars in 2017 officially has been written. After the extremely bright
burst finally faded to black, an international team led by Northwestern
University painstakingly constructed its afterglow — the last bit
of the famed event’s life cycle. 15)

- Not only is the
resulting image the deepest picture of the neutron star
collision’s afterglow to date, it also reveals secrets about the
origins of the merger, the jet it created and the nature of shorter
gamma ray bursts.

- “This is the deepest exposure we have ever taken of this event in visible light,” said Northwestern’s Wen-fai Fong, who led the research. “The deeper the image, the more information we can obtain.”

- The study will be published this
month in The Astrophysical Journal Letters. Fong is an assistant
professor of physics and astronomy in Northwestern’s Weinberg
College of Arts and Sciences and a member of CIERA
(Center for Interdisciplinary Exploration and Research in
Astrophysics), an endowed research center at Northwestern focused on
advancing studies with an emphasis on interdisciplinary connections.

- Many scientists consider the 2017 neutron-star merger, dubbed GW170817, as LIGO’s (Laser Interferometer Gravitational-Wave Observatory)
most important discovery to date. It was the first time that
astrophysicists captured two neutron stars colliding. Detected in both
gravitational waves and electromagnetic light, it also was the
first-ever multi-messenger observation between these two forms of
radiation.

- The light from GW170817 was
detected, partly, because it was nearby, making it very bright and
relatively easy to find. When the neutron stars collided, they emitted
a kilo nova — light 1,000 times brighter than a classical nova,
resulting from the formation of heavy elements after the merger. But it
was exactly this brightness that made its afterglow — formed from
a jet travelling near light-speed, pummeling the surrounding
environment — so difficult to measure.

- “For us to see the
afterglow, the kilonova had to move out of the way,” Fong said.
“Surely enough, about 100 days after the merger, the kilonova had
faded into oblivion, and the afterglow took over. The afterglow was so
faint, however, leaving it to the most sensitive telescopes to capture
it.”

Hubble to the rescue

- Starting in December 2017,
NASA’s Hubble Space Telescope detected the visible light
afterglow from the merger and revisited the merger’s location 10
more times over the course of a year and a half.

Figure 12: The box indicates where the now-faded afterglow was located (image credit: Northwestern University)

- At the end of March 2019,
Fong’s team used the Hubble to obtain the final image and the
deepest observation to date. Over the course of seven-and-a-half hours,
the telescope recorded an image of the sky from where the neutron-star
collision occurred. The resulting image showed — 584 days after
the neutron-star merger — that the visible light emanating from
the merger was finally gone.

- Next,
Fong’s team needed to remove the brightness of the surrounding
galaxy, in order to isolate the event’s extremely faint
afterglow.

- “To accurately measure the
light from the afterglow, you have to take all the other light
away,” said Peter Blanchard, a postdoctoral fellow in CIERA and
the study’s second author. “The biggest culprit is light
contamination from the galaxy, which is extremely complicated in
structure.”

- Fong, Blanchard and their
collaborators approached the challenge by using all 10 images, in which
the kilonova was gone and the afterglow remained as well as the final,
deep Hubble image without traces of the collision. The team overlaid
their deep Hubble image on each of the 10 afterglow images. Then, using
an algorithm, they meticulously subtracted — pixel by pixel
— all light from the Hubble image from the earlier afterglow
images.

- The result: a final time-series of
images, showing the faint afterglow without light contamination from
the background galaxy. Completely aligned with model predictions, it is
the most accurate imaging time-series of GW170817’s visible-light
afterglow produced to date.

- “The brightness evolution
perfectly matches our theoretical models of jets,” Fong said.
“It also agrees perfectly with what the radio and X-rays are
telling us.”

Illuminating information

- With the Hubble’s deep space
image, Fong and her collaborators gleaned new insights about
GW170817’s home galaxy. Perhaps most striking, they noticed that
the area around the merger was not densely populated with star
clusters.

- “Previous studies have
suggested that neutron star pairs can form and merge within the dense
environment of a globular cluster,” Fong said. “Our
observations show that’s definitely not the case for this neutron
star merger.”

- According to the new image, Fong
also believes that distant, cosmic explosions known as short gamma ray
bursts are actually neutron star mergers — just viewed from a
different angle. Both produce relativistic jets, which are like a fire
hose of material that travels near the speed of light. Astrophysicists
typically see jets from gamma ray bursts when they are aimed directly,
like staring directly into the fire hose. But GW170817 was viewed from
a 30-degree angle, which had never before been done in the optical
wavelength.

- “GW170817 is the first time
we have been able to see the jet ‘off-axis,’” Fong
said. “The new time-series indicates that the main difference
between GW170817 and distant short gamma-ray bursts is the viewing
angle.”

- The study was primarily supported
by the National Science Foundation (award numbers AST-1814782 and
AST-1909358) and NASA (award numbers HST-GO-15606.001-A and
SAO-G09-20058A). 16)

• On April 25, 2019, NFS's
(National Science Foundation's) LIGO (Laser Interferometer
Gravitational-Wave Observatory) and the European-based Virgo detector
registered gravitational waves from what appears likely to be a crash
between two neutron stars—the dense remnants of massive stars
that previously exploded. One day later, on April 26, the LIGO-Virgo
network spotted another candidate source with a potentially interesting
twist: it may in fact have resulted from the collision of a neutron
star and black hole, an event never before witnessed.17)18)

- "The universe is keeping us on our
toes," says Patrick Brady, spokesperson for the LIGO Scientific
Collaboration and a professor of physics at the University of
Wisconsin-Milwaukee. "We're especially curious about the April 26
candidate. Unfortunately, the signal is rather weak. It's like
listening to somebody whisper a word in a busy café; it can be
difficult to make out the word or even to be sure that the person
whispered at all. It will take some time to reach a conclusion about
this candidate."

- "NSF's LIGO, in collaboration with
Virgo, has opened up the universe to future generations of scientists,"
says NSF Director France Cordova. "Once again, we have witnessed the
remarkable phenomenon of a neutron star merger, followed up closely by
another possible merger of collapsed stars. With these new discoveries,
we see the LIGO-Virgo collaborations realizing their potential of
regularly producing discoveries that were once impossible. The data
from these discoveries, and others sure to follow, will help the
scientific community revolutionize our understanding of the invisible
universe."

- The discoveries come just weeks after LIGO and Virgo turned back on.
The twin detectors of LIGO—one in Washington and one in
Louisiana—along with Virgo, located at the European Gravitational
Observatory (EGO) in Italy, resumed operations April 1, after
undergoing a series of upgrades to increase their sensitivities to
gravitational waves—ripples in space and time. Each detector now
surveys larger volumes of the universe than before, searching for
extreme events such as smash-ups between black holes and neutron stars.

- "Joining human forces and
instruments across the LIGO and Virgo collaborations has been once
again the recipe of an incomparable scientific month, and the current
observing run will comprise 11 more months," says Giovanni Prodi, the
Virgo Data Analysis Coordinator, at the University of Trento and the
Istituto Nazionale di Fisica Nucleare (INFN) in Italy. "The Virgo
detector works with the highest stability, covering the sky 90 percent
of the time with useful data. This is helping in pointing to the
sources, both when the network is in full operation and at times when
only one of the LIGO detectors is operating. We have a lot of
groundbreaking research work ahead."

- In addition to
the two new candidates involving neutron stars, the LIGO-Virgo network
has, in this latest run, spotted three likely black hole mergers. In
total, since making history with the first-ever direct detection of gravitational waves
in 2015, the network has spotted evidence for two neutron star mergers;
13 black hole mergers; and one possible black hole-neutron star merger.

- When two black holes collide, they
warp the fabric of space and time, producing gravitational waves. When
two neutron stars collide, they not only send out gravitational waves
but also light. That means telescopes sensitive to light waves across
the electromagnetic spectrum can witness these fiery impacts together
with LIGO and Virgo. One such event occurred in August 2017:
LIGO and Virgo initially spotted a neutron star merger in gravitational
waves and then, in the days and months that followed, about 70
telescopes on the ground and in space witnessed the explosive aftermath
in light waves, including everything from gamma rays to optical light
to radio waves.

- In the case of the two recent
neutron star candidates, telescopes around the world once again raced
to track the sources and pick up the light expected to arise from these
mergers. Hundreds of astronomers eagerly pointed telescopes at patches
of sky suspected to house the signal sources. However, at this time,
neither of the sources has been pinpointed.

- "The search for explosive
counterparts of the gravitational-wave signal is challenging due to the
amount of sky that must be covered and the rapid changes in brightness
that are expected," says Brady. "The rate of neutron star merger
candidates being found with LIGO and Virgo will give more opportunities
to search for the explosions over the next year."

Figure 13: How to catch a
gravitational wave. The world’s first captured gravitational
waves were created in a violent collision between two black holes, 1.3
billion light-years away. When these waves passed the Earth, 1.3
billion years later, they had weakened considerably: the disturbance in
spacetime that LIGO measured was thousands of times smaller than an
atomic nucleus (image credit: LIGO)

- The April 25 neutron star
smash-up, dubbed S190425z, is estimated to have occurred about 500
million light-years away from Earth. Only one of the twin LIGO
facilities picked up its signal along with Virgo (LIGO Livingston
witnessed the event but LIGO Hanford was offline). Because only two of
the three detectors registered the signal, estimates of the location in
the sky from which it originated were not precise, leaving astronomers
to survey nearly one-quarter of the sky for the source.

- The possible
April 26 neutron star-black hole collision (referred to as S190426c) is
estimated to have taken place roughly 1.2 billion light-years away. It
was seen by all three LIGO-Virgo facilities, which helped better narrow
its location to regions covering about 1,100 square degrees, or about 3
percent of the total sky.

- "The latest LIGO-Virgo observing
run is proving to be the most exciting one so far," says David H.
Reitze of Caltech, Executive Director of LIGO. "We're already seeing
hints of the first observation of a black hole swallowing a neutron
star. If it holds up, this would be a trifecta for LIGO and
Virgo—in three years, we'll have observed every type of black
hole and neutron star collision. But we've learned that claims of
detections require a tremendous amount of painstaking
work—checking and rechecking—so we'll have to see where the
data takes us."

- LIGO is funded by NSF and operated
by Caltech and MIT, which conceived of LIGO and led the Initial and
Advanced LIGO projects. Financial support for the Advanced LIGO project
was led by the NSF with Germany (Max Planck Society), the U.K. (Science
and Technology Facilities Council) and Australia (Australian Research
Council-OzGrav) making significant commitments and contributions to the
project. Approximately 1,300 scientists from around the world
participate in the effort through the LIGO Scientific Collaboration,
which includes the GEO Collaboration. A list of additional partners is
available at https://my.ligo.org/census.php

- The Virgo Collaboration is
currently composed of approximately 350 scientists, engineers, and
technicians from about 70 institutes from Belgium, France, Germany,
Hungary, Italy, the Netherlands, Poland, and Spain. The European
Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in
Italy, and is funded by Centre National de la Recherche Scientifique
(CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in
Italy, and Nikhef in the Netherlands. A list of the Virgo Collaboration
members can be found at http://public.virgo-gw.eu/the-virgo-collaboration/. More information is available on the Virgo website at http://www.virgo-gw.eu.

• April 26, 2019: In the few
weeks since LIGO kicked off its third observing run, it’s also
already detected three potential black hole collisions and a neutron
star merger, bringing its total lifetime gravitational wave haul to 14.
19)

- It took astronomers a century to
make the first-ever gravitational wave detection, confirming a core
prediction of Albert Einstein’s theory of general relativity. But
this month, the floodgates have opened.

- On Friday (26 April), scientists
with the LIGO announced they’ve likely detected a second
gravitational wave event in as many days. Detectors at three locations
around the world caught the arrival of a probable ripple in space-time
around 11:20 a.m. E.T. (~15:20 GMT). It followed right on the heels of
a gravitational wave detection Thursday that sent astronomers racing to
observe the event with their telescopes.

- In all, it’s the fifth
gravitational wave detection this month. And the influx has astronomers
excited about kickstarting the era of multi-messenger astronomy,
where scientists can combine gravitational wave data with observations
from conventional telescopes to gain new insights into extreme cosmic
events like colliding black holes and neutron stars.

- Scientists suspect
Thursday’s event marked the second-ever gravitational wave
detection of two colliding neutron stars, the collapsed cores left
behind when giant stars go supernova. The merger would have likely
spawned a new black hole. Astronomers spent Thursday searching for any
signs of the collision on the sky. They’re less certain about the
celestial event that led to today’s detection: There’s
about a one in seven chance that it was a false alarm caused by earthly
vibrations. Its signal is right at the threshold of what LIGO can pick
out.

- If this latest signal does turn
out to be real cosmic collision, though, scientists say that
there’s a chance it may be the hallmark of a never-before-seen
event: the collision of a neutron star and a black hole. But odds still
favor it as a third neutron star merger.

• April 26, 2019: For just the
second time, physicists working on the LIGO (Laser Interferometer
Gravitational-Wave Observatory) and at Virgo have caught the
gravitational waves of two neutron stars colliding to likely form a
black hole. 20)21)

- The ripples in space-time traveled
some 500 million light-years and reached the detectors at LIGO, as well
as its Italian sister observatory, Virgo, at around 4 a.m. E.T.
Thursday (~8:00 GMT), 25 April. Team members say there’s a more
than 99 percent chance that the gravitational waves were created from a
binary neutron star merger.

Shot at a Kilonova

- In the moments after the event, a
notice went out alerting astronomers around the world to turn their
telescopes to the heavens in hopes of catching light from the
explosion, called a kilonova. Kilonovae are 1,000 times brighter than
normal novae, and they create huge amounts of heavy elements, like gold
and platinum. That brightness makes it easy for astronomers to find
these events in the night sky — provided they’ve been given
a heads-up and location from LIGO first.

- Scientists use any slight delays
between when signals reach the detectors to help them better
triangulate where the waves originated in the sky. But one of
LIGO’s twin detectors was offline Thursday when the gravitational
wave reached Earth, making it hard for astronomers to triangulate
exactly where the signal was coming from. That sent astronomers racing
to image as many galaxies as they could across a region covering
one-quarter of the sky.

- And instead of finding one
potential binary neutron star merger, astronomers turned up at least
two different candidates. Now the question is which, if any, are
related to the gravitational wave that LIGO saw. Sorting that out will
require more observations, which were already happening around the
world as darkness fell.

- “I would assume that every
observatory in the world is observing this now,” says astronomer
Josh Simon of the Carnegie Observatories. “These two candidates
(they’ve) found are relatively close to the equator, so they can
be seen from both the Northern and Southern Hemisphere.”

- Simon also says that, as of
Thursday afternoon in the United States, telescopes in Europe and
elsewhere should be gathering spectra on these objects. His fellow
astronomers at the Carnegie Observatories turned their telescopes at
Chile’s Las Campanas Observatory to the event Thursday night.

• March 26, 2019: The National
Science Foundation's LIGO (Laser Interferometer Gravitational-Wave
Observatory) is set to resume its hunt for gravitational
waves—ripples in space and time—on April 1, after receiving
a series of upgrades to its lasers, mirrors, and other components.
LIGO—which consists of twin detectors located in Washington and
Louisiana—now has a combined increase in sensitivity of about 40
percent over its last run, which means that it can survey an even
larger volume of space than before for powerful, wave-making events,
such as the collisions of black holes. 22)

- Joining the search will be Virgo,
the European-based gravitational-wave detector, located at the European
Gravitational Observatory (EGO) in Italy, which has almost doubled its
sensitivity since its last run and is also starting up April 1.

- "For this third observational run,
we achieved significantly greater improvements to the detectors'
sensitivity than we did for the last run," says Peter Fritschel, LIGO's
chief detector scientist at MIT. "And with LIGO and Virgo observing
together for the next year, we will surely detect many more
gravitational waves from the types of sources we've seen so far. We're
eager to see new events too, such as a merger of a black hole and a
neutron star."

- In 2015, after LIGO began
observing for the first time in an upgraded program called Advanced
LIGO, it soon made history by making the first direct detection of gravitational waves.
The ripples traveled to Earth from a pair of colliding black holes
located 1.3 billion light-years away. For this discovery, three of
LIGO's key players—Caltech's Barry C. Barish, the Ronald and
Maxine Linde Professor of Physics, Emeritus, and Kip S. Thorne, the
Richard P. Feynman Professor of Theoretical Physics, Emeritus, along
with MIT's Rainer Weiss, professor of physics, emeritus—were awarded the 2017 Nobel Prize in Physics.

- Since then, the
LIGO-Virgo detector network has uncovered nine additional black hole
mergers and one explosive smashup of two neutron stars. That event,
dubbed GW170817, generated not just gravitational waves but light, which was observed by dozens of telescopes in space and on the ground.

- "With our three detectors now
operational at a significantly improved sensitivity, the global
LIGO-Virgo detector network will allow more precise triangulation of
the sources of gravitational waves," says Jo van den Brand of Nikhef
(the Dutch National Institute for Subatomic Physics) and VU University
Amsterdam, who is the spokesperson for the Virgo collaboration. "This
will be an important step toward our quest for multi-messenger
astronomy."

Figure 15: Current operating
facilities in the global network include the twin LIGO
detectors—in Hanford, Washington, and Livingston,
Louisiana— Virgo in Italy and GEO600 in Germany (image credit:
LIGO-Virgo-GEO600 collaboration)

- Now, with the start of the next
joint LIGO-Virgo run, the observatories are poised to detect an even
greater number of black hole mergers and other extreme events, such as
additional neutron-neutron star mergers or a yet-to-be-seen black
hole-neutron star merger. One of the metrics the team uses for
measuring increases in sensitivity is to calculate how far out they can
detect neutron-neutron star mergers. In the next run, LIGO will be able
to see those events out to an average of 550 million light-years away,
or more than 190 million light-years farther out than before.

- A key to
achieving this sensitivity involves lasers. Each LIGO installation
consists of two long arms that form an L shaped interferometer. Laser
beams are shot from the corner of the "L" and bounced off mirrors
before traveling back down the arms and recombining. When gravitational
waves pass by, they stretch and squeeze space itself, making
imperceptibly tiny changes to the distance the laser beams travel and
thereby affecting how they recombine. For this next run, the laser
power has been doubled to more precisely measure these distance
changes, thereby increasing the detectors’ sensitivity to
gravitational waves.

- Other upgrades were made to LIGO's
mirrors at both locations, with a total of five of eight mirrors being
swapped out for better-performing versions. "We had to break the fibers
holding the mirrors and very carefully take out the optics and replace
them," says Calum Torrie, LIGO's mechanical-optical engineering head at
Caltech. "It was an enormous engineering undertaking."

- This next run also includes
upgrades designed to reduce levels of quantum noise. Quantum noise
occurs due to random fluctuations of photons, which can lead to
uncertainty in the measurements and can mask faint gravitational-wave
signals. By employing a technique called "squeezing," initially
developed for gravitational-wave detectors at the Australian National
University, and matured and routinely used since 2010 at the GEO600 detector,
researchers can shift the uncertainty in the photons around, making
their amplitudes less certain and their phases, or timing, more
certain. The timing of photons is what is crucial for LIGO's ability to
detect gravitational waves.

- Torrie says that the LIGO team has
spent months commissioning all of these new systems, making sure
everything is aligned and working correctly. "One of the things that is
satisfying to us engineers is knowing that all of our upgrades mean
that LIGO can now see farther into space to find the most extreme
events in our universe."

• February 27, 2019: LIGO and
Virgo are pleased to announce that the strain data from the O2
observing run have been released. These data are now available through
the Gravitational Wave Open Science Center (https://gw-openscience.org). 23)

- The O2 observing run began on
November 30, 2016 and ended on August 25, 2017. The release includes
over 150 days of data from each of the two LIGO observatories, as well
as 20 days of data from Virgo, making this the largest data set of
"advanced" gravitational wave detectors to date. Observations in O2
include seven binary black hole mergers, as well as the first binary
neutron star merger observed in gravitational waves, all recently
published with the GWTC-1 catalog.
The LIGO Scientific Collaboration and Virgo Collaboration have
published a number of papers based on these data; please see the LIGO Scientific Collaboration web pages
for a list of these papers, and several more will be appearing soon.
Along with the strain data, the release contains detailed documentation
and links to open source software tools.

- O2 is the second observing run of
Advanced LIGO, and the first observing run of Advanced Virgo, which
joined O2 on August 1st, 2017. Data from Advanced LIGO's first
observing run (O1) are already available online, and have been used in
a number of scientific publications, text books, artistic projects, and
classroom activities. As with previous data releases, the O2 data set
should be useful for both scientific investigations and educational
activities.

• February 21, 2019: An
international research team including astronomers from the Max Planck
Institute for Radio Astronomy in Bonn, Germany, has combined radio
telescopes from five continents to prove the existence of a narrow
stream of material, a so-called jet, emerging from the only
gravitational wave event involving two neutron stars observed so far.
With its high sensitivity and excellent performance, the 100 m radio
telescope in Effelsberg played an important role in the observations. 24)

- In August 2017, two neutron stars
were observed colliding, producing gravitational waves that were
detected by the American LIGO and European Virgo detectors. Neutron
stars are ultra-dense stars, roughly the same mass as the Sun, but
similar in size to a city like Cologne. This event is the first and
only one of this type that has been observed so far, and it happened in
a galaxy 130 million light years away from Earth, in the constellation
of Hydra.

- Astronomers
observed the event and the subsequent evolution across the entire
electromagnetic spectrum, from gamma-rays, X-rays to visible light and
radio waves. Two hundred days after the merger, observations combining
radio telescopes in Europe, Africa, Asia, Oceania, and North America
proved the existence of a jet emerging from this violent collision.
These findings are now published in the scientific journal Science by
an international team of astronomers, led by Giancarlo Ghirlanda from
the Italian National Institute for Astrophysics (INAF).

- This neutron star merger
represented the first case where it was possible to associate a
detection of gravitational waves to an object emitting light. The event
has confirmed scientific theories that have been under discussion for
tens of years, and the association of neutron star mergers with one of
the most powerful explosions in the Universe: gamma-ray bursts. After
the merger, a huge amount of material was expelled into space, forming
a shell around the object. Astronomers have been tracing its evolution
at different wavelengths. However, there were still some remaining
questions concerning this event that could not be clarified by any
previous observations.

- "We expected part of the material
to be ejected through a collimated jet, but it was unclear whether this
material could successfully pierce through the surrounding shell."
explains Ghirlanda. “There were two competing scenarios: In one
case, the jet cannot break through the shell, instead generating an
expanding bubble around the object. In the other, the jet is successful
in penetrating the shell and then propagates further into space”,
expands Tiziana Venturi (INAF). Only the acquisition of very sensitive
radio images with very high resolution would discard one scenario or
the other. This required the use of a technique known as very long
baseline interferometry (VLBI) that allows astronomers to combine radio
telescopes all around the Earth.

- The authors of this publication
conducted global observations in the direction of the merger on 12
March 2018 using thirty-three radio telescopes from the European VLBI
Network (that connects telescopes from Spain, the United Kingdom, The
Netherlands, Germany, Italy, Sweden, Poland, Latvia, South Africa,
Russia, and China), e-MERLIN in the UK, the Australian Long Baseline
Array in Australia and New Zealand, and the Very Long Baseline Array in
the USA.

Figure 17: Artist’s
impression of the jet of material launched after the merger of the two
neutron stars (image credit: JIVE, Katharina Immer)

- „Our 100 m radio telescope
in Effelsberg participated in the observations and was a key element,
due to its high sensitivity and excellent performance“, says
Carolina Casadio, a member of the research team from the Max Planck
Institute for Radio Astronomy (MPIfR).

- The data from all telescopes were
sent to JIVE (Joint Institute for VLBI in Europe), The Netherlands,
where the most advanced processing techniques were used to produce an
image with a resolution comparable to resolving a person on the surface
of the Moon. In the same analogy, the expanding bubble would appear
with an apparent size equivalent to a truck on the Moon, whereas a
successful jet would be detected as a much more compact object.

- "Comparing the
theoretical images with the real ones, we find that only a jet could
appear sufficiently compact to be compatible with the observed size.",
explains Om Sharan Salafia from INAF in Italy. The team determined that
this jet contained as much energy as produced by all the stars in our
Galaxy during one year. “And all that energy was contained in a
size smaller than one light year.” says Zsolt Paragi, also from
JIVE.

- ”Within Europe we utilize
the RadioNet consortium for an efficient use of our members’
radio telescopes. The observations described here combine radio
observatories all over Europe and world wide. They require a
well-coordinated effort of the collaborating observatories and
institutions to achieve such exciting results”, explains Anton
Zensus, Director at MPIfR and coordinator of the RadioNet consortium.

- In the coming years, many more of
these neutron star binary mergers will be discovered. “The
obtained results also suggest that more than 10% of all these mergers
should exhibit a successful jet.”, explains Benito Marcote from
JIVE. "These types of observations will allow us to unveil the
processes that take place during and after some of the most powerful
events in the Universe.", concludes Sándor Frey from the Konkoly
Observatory in Hungary.

Figure 18: Image of the source
obtained from the combination of thirty-three radio telescopes from
five continents. The source can be seen in the center of the image as a
red spot (false color image made entirely for illustration), image
credit: Giancarlo Ghirlanda/Science

• December 4, 2018: Scientists with the LIGO and Virgo gravitational wave observatories report four new sets
of these ripples in spacetime. Those additions bring the total count to
11, the researchers say in a study published December 3 at arXiv.org,
marking major progress since the first gravitational wave detection in 2015 (SN: 3/5/16, p. 6). 26)

- All but one of the 11 sets of
waves were stirred up in violent collisions of two black holes. The one
remaining detection, reported in October 2017, instead came from the
smashup of two stellar corpses called neutron stars (SN: 11/11/17, p. 6).

• December 4, 2018: Researchers
from the University of Portsmouth (UoP) have made vital contributions
to the observations of four new gravitational waves, which were
announced this weekend (1 December).27)

- The new results
are from the National Science Foundation’s LIGO (Laser
Interferometer Gravitational-Wave Observatory) and the European-based
VIRGO gravitational-wave detector. The results were announced at the
Gravitational Wave Physics and Astronomy Workshop in College Park,
Maryland, USA.

- Three years ago LIGO made the
first observation of a binary black hole merger. Today, there have been
observations of 11 gravitational-wave signals (10 stellar-mass binary
black hole mergers and one merger of two neutron stars, which are the
dense, spherical remains of stellar explosions).

- These observations are
revolutionizing our understanding of the processes by which high mass
stars (10 – 100 times as heavy as our sun) are formed, how they
evolve, and the method by which black holes are produced.

- The new events are known as GW170729, GW170809, GW170818, and GW170823,
in reference to the dates they were detected. All of the events are
included in a new catalogue, also released Saturday, with some of the
events breaking records.

- Researchers from the newly-formed
Gravitational-Wave Physics Group in the University’s ICG
(Institute of Cosmology and Gravitation) in the University of
Portsmouth have played a significant role in the observation of the
first 11 gravitational-wave events.

- Dr Laura Nuttall, a senior
lecturer in the ICG, made the initial observation of GW170729. This
event, detected in the second observing run on July 29 2017, is the
most massive and distant gravitational-wave source ever observed. Any
theoretical work trying to understand the mechanisms by which black
holes form, now must allow for black holes as massive as this one to be
produced.

- Dr Andrew Lundgren, a reader at
the ICG, was one of the main developers of the noise subtraction
scheme, which was necessary to increase the sensitivity of the LIGO
observatories to be able to confirm that GW170729, GW170809 and
GW170818 were genuine gravitational-wave signals. Without this work, we
would only be talking about eight observed gravitational-wave signals
today. 28)

- Dr Ian Harry, a senior lecturer at
the ICG, is one of the two main developers of the PyCBC algorithm,
which is responsible for searching for merging black holes and neutron
stars in Advanced LIGO and Advanced Virgo data. This code made the
first observation of many of the 11 gravitational-wave signals seen so
far.

- Dr Harry said: “I’m
happy to see the vital contributions our researchers have made to the
observation of the first 11 gravitational-wave events.
Gravitational-wave observations offer us a way to observe astrophysical
sources that have never been seen before, including the collision of
two black holes. These observations allow us to begin to understand the
processes by which black holes are produced and explore the
environments in which they are formed.

- “However, this is only the
beginning of gravitational-wave astronomy, and as our observatories
become more sensitive we expect to observe hundreds of sources in the
coming years. My personal hope is that we observe something truly
unexpected in the next years, which would help us to better understand
the Universe that we live in. Gravitational-wave astronomy is one of
the fastest growing fields in astronomy and collaboration between the
new Gravitational-Wave Physics Group and existing ICG researchers
offers tremendous possibilities for future world-leading
research.”

Figure 19: These observations
allow us to begin to understand the processes by which black holes are
produced and explore the environments in which they are formed (image
credit: UoP Team)

• August 17, 2018: Today LIGO
commemorates the one-year anniversary of its most important discovery
to-date: The detection of a merging pair of neutron stars, aka a BNS
(Binary Neutron Star) merger. 29)

After a 130
million year journey, the gravitational waves generated by these exotic
stars arrived at LIGO’s Hanford and Livingston detectors in the
United States, and the Virgo detector in Italy on 17 August 2017.
Dubbed a ‘kilonova’ (a term coined in 2010 in a paper
wherein it was theorized that a pair of merging neutron stars would
emit light about 1000 times brighter than a classical nova), the
detection also led to a massive explosion of multimessenger astronomy
results gathered by astronomers from all around the globe. LIGO
announced the discovery to the world with papers published on 16
October 2017.

How important was this detection? Well, on the day of the announcement, 84 scientific papers were published about it.

Today, an internet search for
“GW170817” will yield over 110,000 results, all related to
this one event that captured the world scientific community’s
attention (incidentally, a search for “GW150914”,
LIGO’s first detection, yields a mere 80,000 hits).

Why all the Excitement? Up until
that day in August 2017, LIGO’s detections had all been
gravitational waves caused by merging black holes. While there’s
no doubt that those discoveries have been monumental, the scope and
magnitude of this discovery would prove unprecedented. The LIGO and
Virgo detection has become probably the most widely studied
astronomical event in human history. Within days, this object was being
examined by nearly one-third of the world’s electromagnetic (EM)
astronomers. The fact that EM astronomers were able to observe the
phenomenon alongside GW astronomers is what truly elevated this event
to history-making levels. LIGO’s previous detections of merging
black holes did not result in such widespread study because, by their
very nature, black holes are believed not to emit electromagnetic waves
(i.e., light of any wavelength). No amount of searching by astronomers
using telescopes designed to observe EM radiation has revealed
anything. Only gravitational wave observatories like LIGO and Virgo can
‘observe’ black holes colliding.

Neutron stars are different,
however. Unlike black holes, neutron stars are made up of actual
matter, including copious amounts of neutrons (hence their moniker),
and when you accelerate or slam matter together you get electromagnetic
radiation (again, not something one expects to detect from colliding
black holes).

But like black
holes, neutron stars are massive and compact enough to generate
gravitational waves when they collide. This combination of properties
(material composition and density) means that colliding neutron stars
can emit both gravitational waves AND electromagnetic radiation:
“light” in all its forms from gamma rays to radio waves. In
fact, it was a burst of gamma rays, arriving 1.7 seconds after the
gravitational waves, which alerted the broader astronomical community
to something truly extraordinary and led to an unprecedented global
effort to study the phenomenon.

• February 22, 2018: The
National Science Foundation funded Advanced LIGO Documentary Project
released a new video looking at information coming from first
multi-messenger detection of colliding neutron stars. On August 8,
2017, LIGO joined forces with Virgo and over 70 astronomical
observatories to look at a neutron star merger through gravitational
waves and electromagnetic waves (light). This video explores the
significance of that event. 30)

Figure 21: LIGO: A Discovery that
Shook the World. This is the third video in Advanced LIGO Documentary
Project's eight-part series on LIGO's historic discovery of
gravitational waves and the birth of the new age of gravitational wave
astronomy. In August 2017, LIGO and its Italian partner, VIRGO, made a
discovery as important as its historic first detection of gravitational
waves in 2015. They detected gravitational waves from two colliding
neutron stars, which ejected a spectacular gamma ray burst that was
seen by seven space-based telescopes and dozens of astronomical
observatories on earth. It was the long dreamed-of marriage of
gravitational wave astronomy with conventional astronomy, and the
results were spectacular (video credit: Advanced LIGO Documentary
Project)

• December 11, 2017: Physics
World announced that the first multi-messenger detection of a neutron
star merger was 2017's breakthrough of the year. On August 17, 2017
LIGO detected a gravitational wave that was expected to come from a
neutron star merger. Around 2 seconds later a Gamma Ray Burst occurred
and was detected by the Fermi Gamma-ray Space Telescope. Together with
LIGO, Virgo and Fermi's information astronomers were able to piece
together approximately where in the sky the neutron star merger
occurred. Telescopes around the world pointed there scopes at the spot
and soon identified precisely where the neutron stars had collided. In
the coming months more than 70 telescopes observed multiple frequencies
of electromagnetic radiation coming from the neutron star merger,
yielding a treasure trove of information about the kilonova that
occurred when the two neutron stars collided. Over 50 collaborations,
including LIGO participated in this venture. For more on this detection
check out our news article, or the press release. Last year Physics
World awarded LIGO with the 2016 breakthrough of the year, due to the
detection it's detection of gravitational waves. 31)

• October 16, 2017: Another
LIGO gravitational wave detection has spawned an explosion of new
science across the global astronomical community. On August 17, 2017,
the two LIGO instruments (funded by the National Science Foundation)
and its sister facility, Virgo, near Pisa, Italy, sensed tell-tale
signs of the remnant cores of two massive stars spiraling toward and
then smashing into each other some 130 million light years away. The
objects were quickly identified as neutron stars, the collapsed cores
of stars that were once much more massive than our Sun. They are called
“neutron stars” because their matter is so densely packed
it is composed primarily of neutrons. One such star containing as much
matter as our Sun would be just 10 to 15 km in diameter, and a teaspoon
of its material would weigh about one-billion tons on Earth. Using the
signals received in LIGO’s detectors, the masses of the neutron
stars were determined to 1.1 to 1.6 times as massive as our Sun. 32)

- LIGO Hanford Observatory (LHO)
Head, Michael Landry explained what LIGO saw when it made this landmark
discovery: “LIGO and Virgo detected 100 seconds of gravitational
waves as these two neutron stars spiraled together in a massive and
fiery collision,” he said. “In a sprawling follow-up
campaign involving about one-quarter of the world’s professional
astronomers, observatories in space and on the Earth have detected
radiation in all wavelengths from gamma rays to radio waves. But the
LIGO and Virgo detectors were absolutely essential in identifying and
pinpointing the event in the sky, allowing this campaign to
proceed”, Landry added.

- This discovery adds a new way of
learning about the universe through “multi-messenger
astronomy”, where data from traditional telescopes, neutrino
detectors, and now gravitational wave observatories are shared and
compared to glean even deeper insights into the nature of the universe.

- This historic detection came just
three days after another historic detection, LIGO’s fourth, which
was also detected by the Virgo interferometer in Pisa Italy, making it
the first detection by Virgo, and the first three-detector observation
of a gravitational wave. Reveling after that event, LIGO scientists
were astonished to learn of yet another detection, this one completely
different from anything LIGO had seen before.

- Salvatore (Salvo) Vitale,
assistant professor of physics at MIT, was attending a conference in
Amsterdam along with other LIGO scientists, when he first got word of
this second detection in 3 days. The first alert he received included a
‘false alarm rate’ (FAR), a measure of how likely it is
that the event was not real. In this case, the FAR was reported as 3 x
10-12, which is, according to Vitale, “ridiculously low!”

- How ridiculously low? This figure
suggests that the chance that some random but nearly identical bits of
‘noise’ that happened to look like gravitational waves
appeared in the instruments at essentially the same time was less than
1 in 80,000 years.

- Two minutes
after that first alert, the first scan of the event, automatically
generated from the Hanford data, was distributed, and it was distinctly
different from anything LIGO had seen before. Signals of black hole
mergers last just fractions of a second. This signal lasted well in
excess of 30 seconds (in the end, it was shown to have lasted nearly 2
minutes, 500 times longer than black hole mergers). This was a clear
indicator that the objects that created the signal were much less
massive than black holes. To Vitale and everyone else, the unique
properties of the signal could mean only one thing: LIGO had caught its
first gravitational wave from merging neutron stars.

- This was in itself a surprise, as
Vitale explained. “I saw the omega scan from Hanford, and saw
that there was a clear chirp signal, which I remember thinking is
ridiculous, because we never thought we’d see anything in an
omega scan from a binary neutron star merger .... But this [one] was so
loud that we saw it too!”

Figure 22: Top: Thirty-seconds
worth of binary neutron star inspiral as it appeared in the LIGO
detectors. The entire signal lasted 100 seconds. Bottom: LIGO's first
black hole merger detection. The duration of the "chirp" was just 0.2
seconds; 500 times shorter than the signal generated by the neutron
stars (image credit: Caltech/MIT/LIGO Lab)

- At the same time that all this was
happening, LIGO scientists were alerted to another remarkable
astronomical event, which occurred within 2 seconds of LIGO’s
detection. The Fermi gamma ray space telescope had recorded a "short" gamma ray burst (sGRB) just 1.7 seconds after the arrival of the gravitational waves.

- Gamma ray bursts
are seen quite frequently, but what causes them has remained a mystery.
Knowing that neutron star mergers were expected to generate
electromagnetic radiation, likely of very high energy, excitement among
LIGO scientists began to grow as it became more and more plausible that
the first electromagnetic counterpart to a gravitational wave (GW) had
been observed. The time of arrival of the sGRB and GW signals was
especially telling, and important to validating the relationship
between them.

- Vitale explained, “You want
the gamma ray burst to come after the gravitational waves because first
you have to smash the objects together, then the material is warmed up,
and then you get the radiation. So you would expect to see the
gravitational waves first.”

- As the pieces began to fall into place, the magnitude of LIGO’s detection became all the more weighty.

- “Then it was, like ....
‘Okay. Oookay .... let’s take a chair .... and sit down
.....” said Vitale, laughing as he recalled his feelings at that
moment.

- The only way to confirm a
correlation between the GRB and the GW, however, would be to find the
source object on the sky; but there was a problem. At that point, only
the LIGO Hanford data had been processed and distributed; without the
Livingston data, no such localization of a source would be possible.

- Matt Evans (Assistant Professor of
Physics at MIT) recalls the flurry of communication he was receiving in
those early moments.

- “There was this hubbub by
Salvo talking about a signal at LHO that looked like a binary neutron
star coincident with the Fermi alert. But there hadn’t been
anything from Livingston, so there was a moment of doubt of the
validity of the signal.”

- The missing data from Livingston
was puzzling. Reed Essick (Postdoctoral Fellow, UChicago Kavli
Institute for Cosmological Physics) explained: "On the search side,
everything looked good, and a sanity check of the detectors told us
that LLO (LIGO Livingston Observatory) was in science-mode. So why
didn’t the event ‘trigger’ in Livingston?”

- Essick decided to check Livingston
data for ‘glitches’, random bits of loud, sudden noise that
can drown out other signals in the detectors. Running an algorithm
designed specifically for this task, Essick saw that a glitch had in
fact occurred at LLO at the same time that the signal appeared in the
Hanford interferometer. Sifting through the files manually, Essick
found the data stamped with the time of the glitch (and the detection),
and there it was.

- This was why LLO didn’t
automatically send out a trigger alert. The glitch caused LLO’s
computers to disregard, or ‘veto’ that part of the data
stream. Looking at it, it’s no wonder! At first glance, it looked
ugly. However, Evans explained that it really wasn’t as bad as it
seemed.

- “The glitch looks really
terrible on the scan. But the truth is, it’s large in amplitude
and short in time, so it wouldn’t ruin our ability to do any
science on it.”

- Evans added, “Glitches
happen every few hours, so the probability of one landing on top of a
signal is very low. Nevertheless, people have been working on this sort
of possibility for a while, so we were prepared.”

- Most remarkably, despite the size
of the glitch, the gravitational wave signal itself was still clearly
visible (see image of Figure 23). Seeing that was a moment that Vitale remembered vividly.

- “It was a mix of happiness,
tension, and disbelief. We saw that beautiful image of the chirp going
through the glitch and coming out the other side. And at that point, it
was pretty incredible.”

- In a mathematical equivalent of a
game of Operation, at least three teams of people began working on
separating the glitch from the signal. As with everything LIGO-related,
even cleaning a fraction of a second of data required a group effort!
Ultimately, the work paid off. After a few hours, the glitch had been
cleanly removed, and some extraordinary science was about to begin.

Spreading the Word

- In anticipation of this kind of
event, over the years, LIGO had signed agreements with 90 astronomical
observatories around the world to hunt for signs of electromagnetic
(EM) radiation escaping from a gravitational wave event; LIGO would
share sky-coordinates with its partners, who would then start
searching. Until 17 August 2017, no one had found any such
counterparts, but the lure of being among the first ones to detect some
familiar radiation from a gravitational wave event has kept
LIGO’s astronomy partners engaged for over 10 years. This event,
combined with a coincident gamma ray detection, represented the best
chance yet for astronomers to find something, but to do so, LIGO had to
tell them where to look. To that end, LLO data (cleaned of the glitch)
were combined with Hanford and Virgo data, and a sky-map narrowing down
the possible location of the source of the gravitational waves was
generated. The GW map was then merged with Fermi’s GRB map and
another region calculated using the INTEGRAL gamma ray space telescope.
The result was nothing short of amazing.

Figure 23:
The glitch that prevented LIGO Livingston's automated system from
distributing the signal from the merging neutron stars. Despite the
glitch, the curved GW signal is clearly visible. It looked bad at
first, but only 10ms of data needed to be cleaned up (image credit:
Caltech/MIT/LIGO Lab)

- The location of the source of the
gravitational waves as predicted by LIGO-Virgo data sat beautifully
inside the regions of the sky estimated to contain the source of the
gamma ray burst as determined by Fermi and INTEGRAL. The resulting
search area was small enough that within 12 hours LIGO’s optical
astronomy partners had successfully tracked down and imaged a residual
fireball at the edge of a galaxy (NGC4993) some 130 million light years
distant–most of that time was spent waiting for dusk in Chile,
where the first observations could be made. The long-awaited discovery
of an EM counterpart to a gravitational-wave detection had been
confirmed! Furthermore, the answer to another long-sought-after
question was solved: Astronomers could now say with certainty, that at
least one source of short gamma ray bursts in the universe is merging
neutron stars.

Figure 24: The skymap created by
LIGO-Virgo (green) showing the possible location of the source of
gravitational waves, compared with regions containing the location of
the gamma ray burst source from Fermi (purple) and INTEGRAL (grey). The
inset shows the actual position of the galaxy (orange star) containing
the "optical transient" that resulted from the merger of two neutron
stars (image credit: NASA/ESO)

Figure 26: The first image of an
'optical transient' resulting from the merger of two neutron stars, and
the first image of an optical counterpart to a gravitational wave
detection. The box at left shows the host galaxy NGC4993, 130 million
light years distant, as it appeared in a Hubble Space Telescope image
taken April 28, 2017. At right is the same galaxy imaged by the Swope
Telescope just a few hours after the gravitational wave and gamma ray
detections on August 17, 2017. The arrow points to the short-lived
visible fireball that resulted from the merger of two neutron stars in
that galaxy (image credit: Swope Supernova Survey via UC Santa Cruz)

- For LIGO, this optical observation
was important for another reason. The distance to the galaxy, as
determined by astronomers, was wholly consistent with LIGO’s
estimated distance of the source of the gravitational waves. Thus, the
astronomers provided a completely independent verification that
LIGO’s methods for determining the distances to gravitational
wave sources are sound.

- From that moment on, all eyes
were, and continue to be (now two months after the initial detection),
on the skies. LIGOs astronomy partners immediately began observing this
object in every wavelength possible, from gamma rays to visible
radiation to radio waves, as the remnant ‘object’ changes
over time. To date, some 70 of LIGO’s optical astronomy partners
have observed this extraordinary event.

- Today, few would disagree with the
statement that the level of interest in and study of this event is
unprecedented. Within 8 weeks of the detection, over 100 scientific
papers describing the results of follow-up studies were written by
scientists around the world. Dozens of these papers were published on
Monday, October 16th alone, with many more certain to follow in the
months and possibly years to come, making this the most broadly and
intensely studied astronomical event in human history.

Deeper Meaning

Beyond the obvious scientific
importance of this discovery, the importance of this event for the LIGO
Laboratory and the wider collaboration goes much deeper. For many, this
single detection represents the apex of careers, the culmination of
decades of hard work and dedication to LIGO and gravitational wave
science.

Dr. Fred Raab, Associate Director
for Observatory Operations at the LIGO Hanford Observatory, shared what
this latest discovery means to him: “After nearly 30 years of
working toward this discovery, I knew that observing the last minute of
a binary neutron star system would give unprecedented precision in its
parameters. Yet I was unable to continue reading an early paper draft
past where I first saw the number for the chirp mass, a key parameter
of the system. I stared in wonder for minutes at that number, measured
to 4 significant figures for a pair of stars more than 100 million
light years from Earth.”

Raab continued,
“This observation means that LIGO is transitioning now from
studying extreme regions of space-time to extreme states of
matter.”

Mike Zucker, LIGO Systems Scientist, had a similar reaction in those first days after the detection:

“I literally stayed awake for
days after GW170817 watching the [astronomy notices] roll in, marveling
at all the extraordinary implications as the revelations topped each
other one by one. I’m just a detector mechanic, but I consider
this to be the most significant achievement of my career.”

Janeen Romie, LLO’s Detector
Engineering Group Lead, was in her office in Livingston talking to her
husband on the phone when she got the first alert: "I noticed that it
was a binary neutron star merger and I was like, ‘I’ve got
to get off the phone! I’ve got to run down the hall! I’ve
got to find out what’s going on!"

Unable to share the news with anyone outside of LIGO, the only thing Romie could do is run and talk to her colleagues.

"That’s why it was so funny for me,” she laughed, “I hung up on my husband!”

Matt Evans shared what he believes
is most meaningful to the LIGO laboratory in light of this discovery:
“This detection, and especially the triple binary black hole
detection with Virgo are important because they demonstrate that we
(LIGO) are not the only ones claiming to detect gravitational
waves”, he said. “This event solidifies our position in
astronomy, not just physics. Other projects around the world will
benefit greatly. We really can do multi-messenger astronomy, and that
is really meaningful and useful.”

This sentiment was echoed by Essick
and Vitale: “What’s most important is represented by the
‘O’ in LIGO”, said Essick, referring to the fact that
the “O” in LIGO stands for Observatory. “We’ve
been selling the idea that we will detect binary neutron stars for
decades, and now we’ve finally done it. We’ve delivered on
that promise.”

Vitale would agree, “I think
this event brings us a step closer to astronomy", he said. "The
detection of these events ...it’s not just ‘collecting
stamps’ anymore. Now we can do lots of cool stuff.”

Salvatore's feelings ran a bit
deeper still: “Those few days were among the most beautiful days
of my life”, he said. “We kept receiving the circulars from
the astronomers. They’d say, ‘oh, we have a GRB’,
‘oh, we have found an EM counterpart in optical’,
‘oh, we found the galaxy’, ‘we found the
X-rays’, etc. It was ... incredible.”

He paused for a moment; then
continued: “It’s also been sad”, he said. “I
don’t know if I’ll ever live a moment like that again in my
life.” - It's doubtful that anyone at LIGO will.

• October 16, 2017:
LIGO’s latest gravitational wave detection has spawned an
explosion of new science across the global astronomical community. On
August 17, 2017, the two LIGO instruments (funded by the National
Science Foundation) and its sister facility, Virgo,
near Pisa, Italy, sensed tell-tale signs of the remnant cores of two
massive stars spiraling toward and then smashing into each other some
130 million light years away. The objects were quickly identified as
neutron stars, the collapsed cores of stars that were once much more
massive than our Sun. They are called “neutron stars”
because their matter is so densely packed it is composed primarily of
neutrons. One such star containing as much matter as our Sun would be
just 10 to 15 km in diameter, and a teaspoon of its material would
weigh about one-billion tons on Earth. Using the signals received in
LIGO’s detectors, the masses of the neutron stars were determined
to 1.1 to 1.6 times as massive as our Sun. 33)

- LIGO Hanford Observatory (LHO)
Head, Michael Landry explained what LIGO saw when it made this landmark
discovery: “LIGO and Virgo detected 100 seconds of gravitational
waves as these two neutron stars spiraled together in a massive and
fiery collision,” he said. “In a sprawling follow-up
campaign involving about one-quarter of the world’s professional
astronomers, observatories in space and on the Earth have detected
radiation in all wavelengths from gamma rays to radio waves. But the
LIGO and Virgo detectors were absolutely essential in identifying and
pinpointing the event in the sky, allowing this campaign to
proceed”, Landry added.

- This discovery adds a new way of
learning about the universe through “multi-messenger
astronomy”, where data from traditional telescopes, neutrino
detectors, and now gravitational wave observatories are shared and
compared to glean even deeper insights into the nature of the universe.

When It Rains, It Pours

- This historic
detection came just three days after another historic detection,
LIGO’s fourth, which was also detected by the Virgo
interferometer in Pisa Italy, making it the first detection by Virgo,
and the first three-detector observation of a gravitational wave.
Reveling after that event, LIGO scientists were astonished to learn of
yet another detection, this one completely different from anything LIGO
had seen before.

- Salvatore (Salvo) Vitale,
assistant professor of physics at MIT, was attending a conference in
Amsterdam along with other LIGO scientists, when he first got word of
this second detection in 3 days. The first alert he received included a
‘false alarm rate’ (FAR), a measure of how likely it is
that the event was not real. In this case, the FAR was reported as 3x10-12, which is, according to Vitale, “ridiculously low!”

- How ridiculously low? This figure
suggests that the chance that some random but nearly identical bits of
‘noise’ that happened to look like a gravitational waves
appeared in the instruments at essentially the same time was less than
1 in 80,000 years.

- Two minutes after that first
alert, the first scan of the event, automatically generated from the
Hanford data, was distributed, and it was distinctly different from
anything LIGO had seen before. Signals of black hole mergers last just
fractions of a second. This signal lasted well in excess of 30 seconds
(in the end, it was shown to have lasted nearly 2 minutes, 500 times
longer than black hole mergers). This was a clear indicator that the
objects that created the signal were much less massive than black
holes. To Vitale and everyone else, the unique properties of the signal
could mean only one thing: LIGO had caught its first gravitational wave
from merging neutron stars.

- This was in itself a surprise, as
Vitale explained. “I saw the omega scan from Hanford, and saw
that there was a clear chirp signal, which I remember thinking is
ridiculous, because we never thought we’d see anything in an
omega scan from a binary neutron star merger .... But this [one] was so
loud that we saw it too!”

• September 27, 2017: The
Virgo Collaboration and the LIGO Scientific Collaboration have jointly
observed the merger of two black holes.This is the fourth
confirmed detection of a binary black hole merger, and the first
detection made using a network of three interferometers.34)35)

The detected waves—observed on
August 14th, 2017 at 10:30:43 UTC (6:30AM EDT) —were produced by
a pair of black holes with 31 and 25 solar masses. They merged to
produce a spinning black hole of 53 solar masses. Combining the signal
from Virgo with the signal observed in the two LIGO observatories
improved the sky localization of the source by over a factor of 10. 36)

The Virgo and LIGO Scientific
Collaborations have been observing since November 30, 2016 in the
second Advanced Detector Observing Run ‘O2’ , searching for
gravitational-wave signals, first with the two LIGO detectors, then
with both LIGO and Virgo instruments operating together since 1 August
2017. Some promising gravitational-wave candidates have been identified
in data from both LIGO and Virgo during our preliminary analysis, and
we have shared what we currently know with astronomical observing
partners. We are working hard to assure that the candidates are valid
gravitational-wave events, and it will require time to establish the
level of confidence needed to bring any results to the scientific
community and the greater public. We will let you know as soon we have
information ready to share. 37)38)

The detected gravitational
waves—ripples in space and time—were emitted during the
final moments of the merger of two black holes with masses about 31 and
25 times the mass of the sun and located about 1.8 billion light-years
away. The newly produced spinning black hole has about 53 times the
mass of our sun, which means that about 3 solar masses were converted
into gravitational-wave energy during the coalescence.

Figure 27: The GWevent GW170814
observed by LIGO Hanford, LIGO Livingston, and Virgo. Times are shown
from August 14, 2017, 10:30:43 UTC. Top row: SNR time series produced
in low latency and used by the low-latency localization pipeline on
August 14, 2017. The time series were produced by time shifting the
best-match template from the online analysis and computing the
integrated SNR at each point in time. The single-detector SNRs in
Hanford, Livingston, and Virgo are 7.3, 13.7, and 4.4, respectively.
Second row: Time-frequency representation of the strain data around the
time of GW170814. Bottom row: Time-domain detector data (in color), and
90% confidence intervals for waveforms reconstructed from a
morphology-independent wavelet analysis (light gray) and BBH (Binary
Black Hole) models (image credit: LIGO and Virgo Collaboration)

The era of gravitational-wave (GW)
astronomy began with the detection of binary black hole (BBH) mergers,
by the Advanced Laser Interferometer Gravitational-Wave Observatory
(LIGO) detectors, during the first of the Advanced Detector Observation
Runs. 39) Three
detections, GW150914, GW151226, and GW170104, and a lower significance
candidate, LVT151012, have been announced so far. The Advanced Virgo
detector joined the second observation run on August 1, 2017.

• June 15,
2016: A second gravitational wave source was detected by LIGO as
reported on June 15, 2016. The LSC (LIGO Scientific Collaboration) and
the Virgo Collaboration used data from the twin LIGO detectors —
located in Livingston, Louisiana, and Hanford, Washington — to
make the discovery, which is accepted for publication in the journal
Physical Review Letters. 40)41)

From the data of the gravitational
wave event, named GW151226, the researchers concluded the second set of
gravitational waves were produced during the final moments of the
merger of two black holes that were 14 and 8 times the mass of the Sun,
and the collision produced a single, more massive spinning black hole
21 times the mass of the Sun. In comparison, the black holes detected
in September 2015 were 36 and 29 times the Sun’s mass, merging
into a black hole of 62 solar masses.

The inferred component masses are
consistent with values dynamically measured in X-ray binaries, but are
obtained through the independent measurement process of gravitational-
wave detection. Although it is challenging to constrain the spins of
the initial black holes, we can conclude that at least one black hole
had spin greater than 0.2. These recent detections in Advanced
LIGO’s first observing period have revealed a population of
binary black holes that heralds the opening of the field of
gravitational-wave astronomy.

The merger occurred approximately
1.4 billion years ago. The detected signal comes from the last 27
orbits of the black holes before their merger. Based on the arrival
time of the signals—the Livingston detector measured the waves
1.1 milliseconds before the Hanford detector—researchers can
roughly determine the position of the source in the sky.

“GW151226 perfectly matches
our theoretical predictions for how two black holes move around each
other for several tens of orbits and ultimately merge,” said
Alessandra Buonanno of UMD (University of Maryland). “Remarkably,
we could also infer that at least one of the two black holes in the
binary was spinning.”

“It is very significant that
these black holes were much less massive than those observed in the
first detection,” said Gabriela Gonzalez, LSC spokesperson and
professor of physics and astronomy at Louisiana State University.
“Because of their lighter masses compared to the first detection,
they spent more time—about one second—in the sensitive band
of the detectors. It is a promising start to mapping the populations of
black holes in our universe.”

• June 1, 2017: Another
gravitational wave has been detected by the LIGO (Laser Interferometer
Gravitational-wave Observatory). An international team announced the
detection today, while the event itself was detected on January 4,
2017. 42)43)

The
team, including engineers and scientists from Northwestern University
in Illinois, published their results in the journal Physical Review
Letters. 44)

Like the previous two detections,
this one was created by the merging of two black holes. These two were
different sizes from each other; one was about 31.2 solar masses, and
the other was about 19.4 solar masses. The combined 50 solar mass event
caused the third wave, which is named GW170104. The black holes were about 3 billion light years away.

LIGO is showing us that their is a
population of binary black holes out there. “Our handful of
detections so far is revealing an intriguing black hole population we
did not know existed until now,” said Northwestern’s Vicky
Kalogera, a senior astrophysicist with the LSC (LIGO Scientific
Collaboration), which conducts research related to the twin LIGO
detectors, located in the U.S.

“Now we have three pairs of
black holes, each pair ending their death spiral dance over millions or
billions of years in some of the most powerful explosions in the
universe. In astronomy, we say with three objects of the same type you
have a class. We have a population, and we can do analysis.”

This third finding strengthens the
case for the existence of a new class of black holes: binary black
holes that are locked in relationship with each other. It also shows
that these objects can be larger than thought before LIGO detected
them. “It is remarkable that humans can put together a story and
test it, for such strange and extreme events that took place billions
of years ago and billions of light-years distant from us.”
– David Shoemaker, MIT.

“We have further confirmation
of the existence of black holes that are heavier than 20 solar masses,
objects we didn’t know existed before LIGO detected them,”
said David Shoemaker of MIT, spokesperson for the LIGO Scientific
Collaboration . “It is remarkable that humans can put together a
story and test it, for such strange and extreme events that took place
billions of years ago and billions of light-years distant from
us.”

“With the
third confirmed detection of gravitational waves from the collision of
two black holes, LIGO is establishing itself as a powerful observatory
for revealing the dark side of the universe,” said David Reitze
of Caltech, executive director of the LIGO Laboratory and a
Northwestern alumnus. “While LIGO is uniquely suited to observing
these types of events, we hope to see other types of astrophysical
events soon, such as the violent collision of two neutron stars.”

•
In February 2016, gravitational-wave astronomy is going international,
as LIGO India (sometimes referred to as INDIGO) received the green
light recently in the wake of the detection announcement. Set to begin
science operations around 2019, the third LIGO detector will be
constructed in India. This will give LIGO the ‘third
vector’ it was initially envisioned with, allowing researchers to
pin down the source direction in the sky. Other detectors are on the
hunt as well, including VIRGO near Pisa, Italy, GEO600 in Germany, and
KAGRA (Kamioka Gravitational Wave Detector), University of Tokyo,
Japan. 45)

The LISA Pathfinder mission of ESA
also started science operations in late February 2016. Launched on
December 3rd, 2015 from Kourou, French Guiana, LISA Pathfinder
won’t detect gravitational waves. It will, however, pave the way
for a full-up spaceborne gravitational wave detector, eLISA (evolved
Laser Interferometer Space Antenna), set to launch sometime in the
2030s.

• On
February 11, 2016, the LIGO Scientific Collaboration and Virgo
Collaboration announced the first confirmed observation of
gravitational waves from colliding black holes. The gravitational wave
signals were observed by the LIGO's twin observatories on September 14,
2015. This confirms a key prediction of Einstein's theory of general
relativity and provides the first direct evidence that black holes
merge.46)

• Already on 14 September 2015
at 09:50:45 UTC, the LIGO Hanford, WA, and Livingston, LA,
observatories detected the coincident signal GW150914 shown in Figure 29.
The initial detection was made by low-latency searches for generic
gravitational-wave transients and was reported within three minutes of
data acquisition. 47)

Figure 28: The First Observation
of Gravitational Waves. On September 14, 2015, LIGO observed ripples in
the fabric of spacetime. This video narrative tells the story of the
science behind that important detection. (video credit: Caltech)

•
February 11, 2016: For the first time, scientists have observed ripples
in the fabric of spacetime called gravitational waves, arriving at the
earth from a cataclysmic event in the distant universe. This confirms a
major prediction of Albert Einstein’s 1915 general theory of
relativity and opens an unprecedented new window onto the cosmos. 48)

Gravitational waves carry
information about their dramatic origins and about the nature of
gravity that cannot otherwise be obtained. Physicists have concluded
that the detected gravitational waves were produced during the final
fraction of a second of the merger of two black holes to produce a
single, more massive spinning black hole. This collision of two black
holes had been predicted but never observed.

The gravitational waves were
detected on September 14, 2015 at 5:51 a.m. Eastern Daylight Time
(09:51 UTC) by both of the twin Laser Interferometer Gravitational-wave
Observatory (LIGO) detectors, located in Livingston, Louisiana, and
Hanford, Washington, USA. The LIGO Observatories are funded by the
National Science Foundation (NSF), and were conceived, built, and are
operated by Caltech and MIT. The discovery, accepted for publication in
the journal Physical Review Letters, was made by the LIGO Scientific
Collaboration (which includes the GEO Collaboration and the Australian
Consortium for Interferometric Gravitational Astronomy) and the Virgo
Collaboration using data from the two LIGO detectors.

Status of the LIGO / Virgo Facilities

• April 1, 2019: A bit more
than three years after the first, landmark detection of gravitational
waves (GWs), the LIGO and Virgo laser interferometer GW observatories
today kick off their third observation run. 49)

- Known in the GW community simply
as “O3,” the year-long observation run will likely yield a
bumper crop of new astronomical observations—the result of a 40
percent improvement in the already jaw-dropping sensitivity
of the two LIGO facilities in the United States and a near-doubling of
the sensitivity of the Virgo facility in Italy. The O3 period also
could see the long-awaited on-streaming of the KAGRA GW observatory in
Japan. And, in a new twist, the LIGO/Virgo Scientific Collaboration
(LSC) will be making data about possible GW detections publicly
available in near realtime.

Boosting laser power

- The O3 run will add to the
impressive string of milestones achieved in the first two GW
observation runs. These include the detection of gravitational waves
from ten binary black-hole mergers, as well as from the collision of a
pair of ultra-dense neutron stars. The latter
detection—coordinated with observations from more traditional
optical, X-ray and gamma-ray telescopes in a path-breaking example of “multi-messenger astronomy”—resulted in a breathtaking harvest of new scientific information.

- LSC scientists are confident that
the LIGO and Virgo observatories will log observations at an even
faster clip in O3, as a result of technical improvements implemented
since the end of the last observation run, O2, in August 2017.

- The improvements include a
doubling of the power of the facilities’ lasers, which in these
observatories are amplified in two Fabry–Pérot cavities 3
to 4 km long that form the arms of a gigantic, L-shaped
Michelson-interferometer. Also installed in the upgrade round were
scattered-light suppressors, or “baffles,” designed to
control stray light within the huge interferometers.

Hammering down noise

- In addition to laser power, other
recent upgrades have centered on efforts to boost sensitivity by
ferreting out and eliminating noise sources in a range of subsystems.

- At LIGO, this has included the
huge engineering challenge of swapping out a number of the 40-kg
mirrors, or test masses, exquisitely suspended at either end of the
laser interferometer arms. As a passing gravitational wave ripples
through spacetime, tiny movements in these hefty mirrors result in
infinitesimal changes of the interferometer arms’ lengths, which
are read as picowatt-scale power fluctuations at the dark port of the
interferometer. The new, better-performing versions of the mirrors
include improved coatings to diminish thermal noise.

Figure 30: Engineers Hugh Radkins
and Betsy Weaver at work inside the vacuum system of the detector at
LIGO Hanford Observatory, Washington, USA, during recent hardware
upgrades. With the upgrades complete, the two U.S. LIGO facilities, in
partnership with the Virgo facility in Italy, are now poised to begin
third observation run (image credit: LIGO/Caltech/MIT/Jeff Kissel)

- At Virgo, meanwhile, the steel
wires suspending the main mirrors have been replaced with fused-silica
versions that quiet down vibrational noise and extend the
facility’s ability to pick up low- and medium-frequency GWs. And
during O3, both LIGO and Virgo will use a trick of quantum mechanics,
the injection of a “squeezed” state of light at the
photodetector, to narrow down the uncertainties in photon arrival times
attributable to fluctuations in the quantum vacuum.

- These and other
technical improvements were partly developed and matured at yet another
facility, GEO 600, a smaller GW observatory in Europe that has served
as a vital testbed for technologies to sharpen the observing power of
the larger sites. GEO 600 will also participate in the O3 run.

Sampling more of the cosmos

- The recent sensitivity upgrades
will enable the global GW network to sample a much-expanded slice of
the cosmos for evidence of high-energy astronomical events. In the O3
run, for example, LIGO’s sensitivity in the wake of the recent
upgrades should enable it to sniff out binary neutron-star mergers to a
distance of 550 million light-years—more than 190 million
light-years further than in O2.

- That, coupled with an eightfold
expansion of the volume of space now visible to Virgo, could increase
the rate of detection of binary black-hole collisions to anywhere from
a few events per month to a few per week, and binary neutron-star
mergers to between one per year and one per month. There’s also
the possibility of picking up more exotic, previously inaccessible
events, such as the merger of a black hole and a neutron star.

Instant access to data

- In another change, the public will
have near-immediate access to this harvest of discoveries, through new
software developed by LSC scientists. The software will be “able
to send open public alerts within five minutes” after a GW
detection, according to Sarah Antier, a postdoctoral research associate
at the Université Paris Diderot, France.

- That will allow rapid public
access to parameters such as type of signal, sky position and estimated
distance for a given GW event. Those parameters, in turn, will let both
professional and amateur astronomers looking at various slices of the
electromagnetic spectrum quickly train their instruments on the right
patch of sky to follow up on the GW observation.

• February 18, 2019: The
National Science Foundation (NSF) is awarding Caltech and MIT $20.4
million to upgrade the Laser Interferometer Gravitational-wave
Observatory (LIGO), an NSF-funded project that made history in 2015
after making the first direct detection of ripples in space and time,
called gravitational waves. 50)51)

- The investment is part of a joint
international effort in collaboration with UK Research and Innovation
and the Australian Research Council, which are contributing additional
funds. While LIGO is scheduled to turn back on this spring, in its
third run of the "Advanced LIGO" phase, the new funding will go toward "Advanced LIGO Plus."

- Advanced LIGO Plus is expected to
commence operations in 2024 and to increase the volume of deep space
the observatory can survey by as much as seven times.

- "With it we expect to detect
gravitational waves from black hole mergers on a daily basis, greatly
increasing our understanding of this dark sector of the universe.
Gravitational-wave observations of neutron star collisions, now very
rare, will become much more frequent, allowing us to more deeply probe
the structure of their exotic interiors."

- Since LIGO's first detection of
gravitational waves from the violent collision of two black holes, it
has observed nine additional black hole mergers and one collision of
two dense, dead stars called neutron stars.

- The neutron star merger gave off
not just gravitational waves but light waves, detected by dozens of
telescopes in space and on the ground. The observations confirmed that
heavy elements in our universe, such as platinum and gold, are created
in neutron star smashups like this one.

- "This award ensures that NSF's
LIGO, which made the first historic detection of gravitational waves in
2015, will continue to lead in gravitational-wave science for the next
decade," said Anne Kinney, assistant director for NSF's Mathematical
and Physical Sciences Directorate, in a statement.

- "With improvements to the
detectors - which include techniques from quantum mechanics that refine
laser light and new mirror coating technology - the twin LIGO
observatories will significantly increase the number and strength of
their detections. Advanced LIGO Plus will reveal gravity at its
strongest and matter at its densest in some of the most extreme
environments in the cosmos. These detections may reveal secrets from
inside supernovae and teach us about extreme physics from the first
seconds after the universe's birth."

- Michael Zucker, the Advanced LIGO
Plus leader and co-principal investigator, and a scientist at the LIGO
Laboratory, operated by Caltech and MIT, said, "I'm thrilled that NSF,
UK Research, and Innovation and the Australian Research Council are
joining forces to make this key investment possible. Advanced LIGO has
altered the course of astrophysics with 11 confirmed gravitational-wave
events over the last three years. Advanced LIGO Plus can expand LIGO's
horizons enough to capture this many events each week, and it will
enable powerful new probes of extreme nuclear matter as well as Albert
Einstein's general theory of relativity."

• November 20, 2018: The LIGO
Laboratory congratulates Derek Davis of Syracuse University and T.J.
Massinger of Caltech for winning the first LIGO Laboratory Award for
Excellence in Detector Characterization and Calibration. 52)

- The LIGO gravitational wave
detectors have registered gravitational wave signals from multiple
black hole mergers and the spectacular collision of two neutron stars
since Advanced LIGO first began observing in 2015. Davis and
Massinger’s work to reduce the noise present in LIGO detector
data was key to making these discoveries possible by allowing searches
to more easily distinguish the signatures of true astrophysical
gravitational wave events in noisy detector data.

- By improving how deep in space the
LIGO-Hanford detector could sense by up to 50%, at least three
gravitational wave signals were confidently detected during Advanced
LIGO's second observing run (O2) that would not have been otherwise.
Their efforts are an outstanding example of the detector
characterization work needed to lay the groundwork for future
discoveries in gravitational wave astrophysics and multi-messenger
astronomy.

- Davis and
Massinger will share a $1000 prize and are invited to present colloquia
at one of the the LIGO Laboratory sites (LIGO-Hanford, LIGO-Livingston,
Caltech, or MIT) to share their achievements with LIGO Laboratory
members. They will each receive an award certificate at the LIGO-Virgo
Collaboration meeting in March 2019.

- Derek Davis is currently a Ph.D.
student at Syracuse University. As a part of the LIGO Scientific
Collaboration, they serve as the Event Validation Lead for the LIGO
Detector Characterization group, leading follow-up investigations of
candidate gravitational-wave detections.

- T.J. Massinger is currently a
postdoctoral scholar at Caltech. He earned his PhD in 2016 from
Syracuse University. Within the LIGO Detector Characterization group,
he serves as instrument science lead and as a liaison to the compact
binary coalescence data analysis working group.

The information compiled and edited in this article was provided byHerbert
J. Kramer from his documentation of: ”Observation of the Earth
and Its Environment: Survey of Missions and Sensors” (Springer
Verlag) as well as many other sources after the publication of the 4th
edition in 2002. - Comments and corrections to this article are always
welcome for further updates (herb.kramer@gmx.net)